I'm thinking you want to put the effect of the tags the Necromancer secret adds to the target to the creature you want to have innate necromancy.
[SYNDROME]
[CE_DISPLAY_TILE:TILE:165:5:0:1:START:0]
[CE_DISPLAY_NAME:NAME:necromancer:necromancers:necromantic:START:0]
This is the syndrome
[CE_ADD_TAG:NOEXERT:NO_AGING:NO_EAT:NO_DRINK:NO_SLEEP:NO_PHYS_ATT_GAIN:NO_PHYS_ATT_RUST:START:0]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Animate corpse]
[CDI:INTERACTION:EXAMPLE RAISE]
[CDI:TARGET:A:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:A:10]
[CDI:VERB:gesture:gestures:NA]
[CDI:TARGET_VERB:shudder and begin to move:shudders and begins to move]
[CDI:WAIT_PERIOD:10]
This is an example of spitting (also a creature interaction):
[CAN_DO_INTERACTION:MATERIAL_EMISSION]
[CDI:ADV_NAME:Spit]
[CDI:USAGE_HINT:NEGATIVE_SOCIAL_RESPONSE]
[CDI:USAGE_HINT:TORMENT]
[CDI:BP_REQUIRED:BY_CATEGORY:MOUTH]
[CDI:MATERIAL:LOCAL_CREATURE_MAT:SPIT:LIQUID_GLOB]
[CDI:VERB:spit:spits:NA]
[CDI:TARGET:C:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:C:15]
[CDI:MAX_TARGET_NUMBER:C:1]
[CDI:WAIT_PERIOD:30]
Now you CAN do as you suggested and give them to humans as a lower probability caste or you can just make a duplicated 'human necromancer' creature and give them an entity that only allows wanderers and is bonecarn or something to kill them off in the worldgen.
Then again it may be simpler to just make a copy of the "meat" material and give it necromancy as an ingested syndrome making this a product of an adventurer mode reaction (meat-> necromancers meat) would make it so that only player adventurers would use it and would also allow for you to have elf/goblin/dwarf/kobold/etc. necromancers whenever you wanted them

.
This probably needs to be broken into its compenent parts (tags) and just added in manually like all other tags:
[CE_ADD_TAG:NOEXERT:NO_AGING:NO_EAT:NO_DRINK:NO_SLEEP:NO_PHYS_ATT_GAIN:NO_PHYS_ATT_RUST:START:0]
This is the Attribute buff from the vampirism example:
[CE_PHYS_ATT_CHANGE:STRENGTH:200:0:AGILITY:200:0:TOUGHNESS:200:0:START:0]
This is NOT max attributes, but it can be used in syndromes which could be convenient depending on the route you use.