Were creatures already have material based weaknesses built it, and take more or less force from certain materials.
Trouble is which material is randomly chosen, just like the creature.
So in order to make sure Were beasts are affected by Silver, you'll need to set night creatures to 0 in world gen, and provide custom interactions for your werebeasts.
The degree that it effects them can be changed as well, and you can adjust their resistance to other materials.
The code that handles this are the lines:
[MATERIAL_FORCE_MULTIPLIER:INORGANIC:SILVER:5:1]
[GENERAL_MATERIAL_FORCE_MULTIPLIER:1:8]
The first line allows you to specify a material, while the second line affects all materials.
The example code makes any silver object hit the creature with 5 times as much force, while any other material hits with 1/8 the normal force.
Here is the example code from the wiki on were creatures, outlining the creature and interaction that needs to be made to replace random generation. Note you'll need to make one of these for each were creature type you make.
[OBJECT:CREATURE]
[CREATURE:NIGHT_CREATURE_32]
[NAME:wereskink:wereskinks:wereskink]
[CASTE_NAME:wereskink:wereskinks:wereskink]
[GENERATED]
[ATTACK_TRIGGER:20:0:0]
[NIGHT_CREATURE_HUNTER]
[CAN_LEARN]
[CAN_SPEAK]
[NO_GENDER]
[BONECARN]
[CRAZED]
[PHYS_ATT_RANGE:STRENGTH:1000:1150:1250:1500:2000:2250:2500]
[PHYS_ATT_RANGE:AGILITY:1000:1150:1250:1500:2000:2250:2500]
[PHYS_ATT_RANGE:TOUGHNESS:850:900:950:1000:1050:1100:1150]
[PHYS_ATT_RANGE:ENDURANCE:850:900:950:1000:1050:1100:1150]
[PHYS_ATT_RANGE:RECUPERATION:450:1050:1150:1250:1350:1550:2250]
[PHYS_ATT_RANGE:DISEASE_RESISTANCE:700:1300:1400:1500:1600:1800:2500]
[MENT_ATT_RANGE:ANALYTICAL_ABILITY:1250:1500:1750:2000:2500:3000:5000]
[MENT_ATT_RANGE:FOCUS:1250:1500:1750:2000:2500:3000:5000]
[MENT_ATT_RANGE:WILLPOWER:1250:1500:1750:2000:2500:3000:5000]
[MENT_ATT_RANGE:PATIENCE:0:333:666:1000:2333:3666:5000]
[MENT_ATT_RANGE:MEMORY:1250:1500:1750:2000:2500:3000:5000]
[MENT_ATT_RANGE:LINGUISTIC_ABILITY:450:1050:1150:1250:1350:1550:2250]
[MENT_ATT_RANGE:MUSICALITY:0:333:666:1000:2333:3666:5000]
[MENT_ATT_RANGE:SOCIAL_AWARENESS:700:1300:1400:1500:1600:1800:2500]
[PERSONALITY:BASHFUL:0:0:0]
[PERSONALITY:STRESS_VULNERABILITY:0:0:0]
[PERSONALITY:FRIENDLINESS:0:0:0]
[PERSONALITY:DISDAIN_ADVICE:100:100:100]
[PERSONALITY:CHEER_PROPENSITY:0:0:0]
[PERSONALITY:GRATITUDE:0:0:0]
[PERSONALITY:TRUST:0:0:0]
[PERSONALITY:ALTRUISM:0:0:0]
[PERSONALITY:CRUELTY:100:100:100]
[NATURAL_SKILL:WRESTLING:6]
[NATURAL_SKILL:BITE:6]
[NATURAL_SKILL:GRASP_STRIKE:6]
[NATURAL_SKILL:STANCE_STRIKE:6]
[NATURAL_SKILL:MELEE_COMBAT:6]
[NATURAL_SKILL:DODGING:6]
[NATURAL_SKILL:SITUATIONAL_AWARENESS:6]
[NATURAL_SKILL:SNEAK:20]
[DIFFICULTY:1]
[LAIR:SIMPLE_BURROW:50]
[NO_DRINK]
[NO_EAT]
[NO_SLEEP]
[BODY_APPEARANCE_MODIFIER:HEIGHT:90:95:98:100:102:105:110]
[BODY_APPEARANCE_MODIFIER:BROADNESS:90:95:98:100:102:105:110]
[LARGE_PREDATOR]
[EVIL]
[SUPERNATURAL]
[FANCIFUL]
[AMPHIBIOUS]
[PETVALUE:2000]
[GRASSTRAMPLE:20]
[BUILDINGDESTROYER:2]
[ALL_ACTIVE]
[SWIMS_INNATE]
[TRAPAVOID]
[NOPAIN]
[NOSTUN]
[NONAUSEA]
[NOFEAR]
[NOEXERT]
[NO_DIZZINESS]
[NO_FEVERS]
[MATERIAL_FORCE_MULTIPLIER:INORGANIC:STEEL:10:1]
[GENERAL_MATERIAL_FORCE_MULTIPLIER:1:2]
[LARGE_PREDATOR]
[SPHERE:ANIMALS]
[SPHERE:CHAOS]
[SPHERE:MOON]
[SPHERE:NIGHT]
[BODY_SIZE:0:0:83000]
[CRTEATURE_TILE:165]
[BODY:RCP_UPPER_BODY:RCP_LOWER_BODY:RCP_NECK:RCP_HEAD:RCP_TWO_PART_ARMS:RCP_TWO_PART_LEGS:RCP_TAIL:RCP_4_FINGERS:RCP_4_TOES:RCP_2_EYES:RCP_LUNGS:RCP_HEART:RCP_GUTS:RCP_THROAT:RCP_SPINE:RCP_UPPER_SPINE:RCP_BRAIN:RCP:SKULL:RCP_MOUTH:RCP_TONGUE:RCP_TEETH:RCP_RIBS:RCP_2_EYELIDS]
[CANOPENDOORS]
[EQUIPS]
[BODY_DETAIL_PLAN:STANDARD_MATERIALS]
[REMOVE_MATERIAL:SKIN]
[REMOVE_MATERIAL:HAIR]
[USE_MATERIAL_TEMPLATE:SCALE:SCALE_TEMPLATE]
[BODY_DETAIL_PLAN:STANDARD_TISSUES]
[REMOVE_TISSUE:SKIN]
[REMOVE_TISSUE:HAIR]
[USE_TISSUE_TEMPLATE:SCALE:SCALE_TEMPLATE]
[BODY_DETAIL_PLAN:VERTEBRATE_TISSUE_LAYERS:SCALE:FAT:MUSCLE:BONE:CARTILAGE]
[BODY_DETAIL_PLAN:STANDARD_HEAD_POSITIONS]
[BODY_DETAIL_PLAN:HUMANOID_HEAD_POSITIONS]
[BODY_DETAIL_PLAN:HUMANOID_RIBCAGE_POSITIONS]
[BODY_DETAIL_PLAN:HUMANOID_RELSIZES]
[USE_MATERIAL_TEMPLATE:SINEW:SINEW_TEMPLATE]
[TENDRONS:LOCAL_CREATURE_MAT:SINEW:200]
[LIGAMENTS:LOCAL_CREATURE_MAT:SINEW:200]
[HAS_NERVES]
[NOBREATHE]
[HOMEOTHERM:10040]
[SELECT_MATERIAL:SCALE]
[STATE_COLOR:ALL_SOLID:CHARCOAL]
[NO_UNIT_TYPE_COLOR]
[COLOR:6:0:0]
[SELECT_MATERIAL:EYE]
[STATE_COLOR:ALL:BLUE]
[GLOWTILE:'"']
[GLOWCOLOR:1:0:1]
[SELECT_TISSUE_LAYER:HEART:BY_CATEGORY:HEART]
[PLUS_TISSUE_LAYER:SCALE:BY_CATEGORY:THROAT]
[TL_MAJOR_ARTERIES]
[USE_MATERIAL_TEMPLATE:BLOOD:BLOOD_TEMPLATE]
[BLOOD:LOCAL_CREATURE_MAT:BLOOD:LIQUID]
[CREATURE_CLASS:GENERAL_POISON]
[USE_MATERIAL_TEMPLATE:CLAW:NAIL_TEMPLATE]
[USE_TISSUE_TEMPLATE:CLAW:CLAW_TEMPLATE]
[TISSUE_LAYER:BY_CATEGORY:FINGER:CLAW:FRONT]
[TISSUE_LAYER:BY_CATEGORY:TOE:CLAW:FRONT]
[ATTACK:PUNCH:BODYPART:BY_TYPE:GRASP]
[ATTACK_SKILL:GRASP_STRIKE]
[ATTACK_VERB:punch:punches]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PREPARE_AND_RECOVER:2:2]
[ATTACK_FLAG_WITH]
[ATTACK_PRIORITY:SECOND]
[ATTACK:KICK:BODYPART:BY_TYPE:STANCE]
[ATTACK_SKILL:STANCE_STRIKE]
[ATTACK_VERB:kick:kicks]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PREPARE_AND_RECOVER:2:2]
[ATTACK_FLAG_WITH]
[ATTACK_PRIORITY:SECOND]
[ATTACK_FLAG_BAD_MULTIATTACK]
[ATTACK:BITE:CHILD_BODYPART_GROUP:BY_CATEGORY:HEAD:BY_CATEGORY:TOOTH]
[ATTACK_SKILL:BITE]
[ATTACK_VERB:bite:bites]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PENETRATION_PERC:100]
[ATTACK_PREPARE_AND_RECOVER:2:2]
[ATTACK_FLAG_EDGE]
[ATTACK_PRIORITY:MAIN]
[ATTACK_FLAG_CANLATCH]
[SPECIALATTACK_INTERACTION:DEITY_CURSE_WEREBEAST_10_BITE]
[ATTACK:FSCRATCH:BY_TYPE:GRASP:BY_CATEGORY:ALL:CLAW]
[ATTACK_SKILL:GRASP_STRIKE]
[ATTACK_VERB:scratch:scratches]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PENETRATION_PERC:100]
[ATTACK_PREPARE_AND_RECOVER:2:2]
[ATTACK_FLAG_EDGE]
[ATTACK_PRIORITY:SECOND]
[ATTACK:TSCRATCH:BY_TYPE:STANCE:BY_CATEGORY:ALL:CLAW]
[ATTACK_SKILL:STANCE_STRIKE]
[ATTACK_VERB:scratch:scratches]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PENETRATION_PERC:100]
[ATTACK_PREPARE_AND_RECOVER:2:2]
[ATTACK_FLAG_EDGE]
[ATTACK_PRIORITY:SECOND]
[ATTACK_FLAG_BAD_MULTIATTACK]
[GAIT:SWIM:Maximum Swim Speed:725:10:3:2175:50:LAYERS_SLOW:STRENGTH:AGILITY:STEALTH_SLOWS:50]
[GAIT:SWIM:Faster Swim:1450:5:3:2175:10:LAYERS_SLOW:STRENGTH:AGILITY:STEALTH_SLOWS:20]
[GAIT:SWIM:Fast Swim:2175:NO_BUILD_UP:5:LAYERS_SLOW:STRENGTH:AGILITY:STEALTH_SLOWS:10]
[GAIT:SWIM:Swim:2900:NO_BUILD_UP:0]
[GAIT:SWIM:Slow Swim:3900:NO_BUILD_UP:0]
[GAIT:SWIM:Creeping Swim:5900:NO_BUILD_UP:0]
[GAIT:WALK:Sprint:212:10:3:637:50:LAYERS_SLOW:STRENGTH:AGILITY:STEALTH_SLOWS:50]
[GAIT:WALK:Run:425:5:3:637:10:LAYERS_SLOW:STRENGTH:AGILITY:STEALTH_SLOWS:20]
[GAIT:WALK:Jog:600:NO_BUILD_UP:5:LAYERS_SLOW:STRENGTH:AGILITY:STEALTH_SLOWS:10]
[GAIT:WALK:Walk:850:NO_BUILD_UP:0]
[GAIT:WALK:Stroll:1800:NO_BUILD_UP:0]
[GAIT:WALK:Creep:2750:NO_BUILD_UP:0]
[GAIT:CRAWL:Scramble:225:10:3:675:50:LAYERS_SLOW:STRENGTH:AGILITY:STEALTH_SLOWS:50]
[GAIT:CRAWL:Faster Crawl:450:5:3:675:10:LAYERS_SLOW:STRENGTH:AGILITY:STEALTH_SLOWS:20]
[GAIT:CRAWL:Fast Crawl:675:NO_BUILD_UP:5:LAYERS_SLOW:STRENGTH:AGILITY:STEALTH_SLOWS:10]
[GAIT:CRAWL:Crawl:900:NO_BUILD_UP:0]
[GAIT:CRAWL:Slow Crawl:1900:NO_BUILD_UP:0]
[GAIT:CRAWL:Creep:2900:NO_BUILD_UP:0]
[GAIT:CLIMB:Scramble:225:10:3:675:50:LAYERS_SLOW:STRENGTH:AGILITY:STEALTH_SLOWS:50]
[GAIT:CLIMB:Faster Climb:450:5:3:675:10:LAYERS_SLOW:STRENGTH:AGILITY:STEALTH_SLOWS:20]
[GAIT:CLIMB:Fast Climb:675:NO_BUILD_UP:5:LAYERS_SLOW:STRENGTH:AGILITY:STEALTH_SLOWS:10]
[GAIT:CLIMB:Climb:900:NO_BUILD_UP:0]
[GAIT:CLIMB:Slow Climb:1900:NO_BUILD_UP:0]
[GAIT:CLIMB:Creep:2900:NO_BUILD_UP:0]
[DESCRIPTION:A large skink twisted into humanoid form. It is crazed for blood and flesh. Its eyes glow blue. Its charcoal scales are blocky and close-set. Now you will know why you fear the night.]
[OBJECT:INTERACTION]
[INTERACTION:DEITY_CURSE_WEREBEAST_10_BITE]
[GENERATED]
[I_SOURCE:ATTACK]
[IS_HIST_STRING_1: bit ]
[IS_HIST_STRING_2:, passing on the skink monster curse]
[I_TARGET:A:CREATURE]
[IT_LOCATION:CONTEXT_CREATURE]
[IT_REQUIRES:CAN_LEARN]
[IT_REQUIRES:HAS_BLOOD]
[IT_FORBIDDEN:NOT_LIVING]
[IT_FORBIDDEN:SUPERNATURAL]
[IT_CANNOT_HAVE_SYNDROME_CLASS:WERECURSE]
[IT_CANNOT_HAVE_SYNDROME_CLASS:VAMPCURSE]
[IT_CANNOT_HAVE_SYNDROME_CLASS:DISTURBANCE_CURSE]
[IT_CANNOT_HAVE_SYNDROME_CLASS:RAISED_UNDEAD]
[IT_CANNOT_HAVE_SYNDROME_CLASS:RAISED_GHOST]
[IT_CANNOT_HAVE_SYNDROME_CLASS:GHOUL]
[I_EFFECT:ADD_SYNDROME]
[IE_TARGET:A]
[IE_IMMEDIATE]
[SYNDROME]
[SYN_CONCENTRATION_ADDED:1000:0]
[SYN_CLASS:WERECURSE]
[CE_BODY_TRANSFORMATION:START:16800:ABRUPT]
[CE:CREATURE:NIGHT_CREATURE_32:DEFAULT]
[CE:PERIODIC:MOON_PHASE:27:0]