I can say that being a werecreature is definitely very fun during the full moon. I'm not sure if this affects all werecreatures, but my wereelk adventurer apparently had [NO_PAIN] in were form, since bandits broke every single bone in my body without me feeling it. I even managed to bite them all to death after my bones were pulverized. I can also say that the material weaknesses/strengths of werecreatures do NOT affect your human form. I found this out the hard way when a bandit smashed the face in of my weak-against-silver wereelk companion while he was in dwarf form using a bronze mace.
As for making werecreatures able to transform at will, you could do it with raw modding, but you'd have to create your own custom werecreature in the raws, or at least copy-paste one of the default ones from the world save.
Here's the example werebeast interaction edited to allow at-will transformations. It requires that there be a creature called WEREBEAST with a caste called DEFAULT in the raws, although you can change it to any creature and caste you want. You can also tweak various values to get it the way you want.
interaction_werebeast
[OBJECT:INTERACTION]
[INTERACTION:WEREBEAST_CURSE]
[I_SOURCE:DEITY]
[IS_USAGE_HINT:MAJOR_CURSE]
[IS_HIST_STRING_1: cursed ]
[IS_HIST_STRING_2: to assume the form of a wolf-like monster every full moon]
[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]
[I_EFFECT:ADD_SYNDROME]
[IE_TARGET:A]
[IE_IMMEDIATE]
[SYNDROME]
[IE_ARENA_NAME:Werewolf]
[SYN_CLASS:WERECURSE]
[CE_BODY_TRANSFORMATION:START:0]
[CE:CREATURE:WEREBEAST:DEFAULT]
[CE:PERIODIC:MOON_PHASE:27:0]
[CE_ADD_TAG:NO_AGING:START:0]
[CE_CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Transform into a werebeast]
[CDI:INTERACTION:WEREBEAST_CHANGE]
[CDI:TARGET:A:SELF_ONLY]
[CDI:VERB:concentrate:concentrates:NA]
[CDI:TARGET_VERB:begin to change:begins to change]
[CDI:WAIT_PERIOD:1000] // This is how long you have to wait before you can change again. It seems this number uses different units than the START and END tokens, because even with a value of 50 I was able to use an interaction every 3 turns as an adventurer.
[INTERACTION:WEREBEAST_BITE]
[I_SOURCE:ATTACK]
[IS_HIST_STRING_1: bit ]
[IS_HIST_STRING_2:, passing on the werebeast 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]
[I_EFFECT:ADD_SYNDROME]
[IE_TARGET:A]
[IE_IMMEDIATE]
[SYNDROME]
[SYN_CLASS:WERECURSE]
[CE_BODY_TRANSFORMATION:START:16800]
[CE:CREATURE:WEREBEAST:DEFAULT]
[CE:PERIODIC:MOON_PHASE:27:0]
[CE_CAN_DO_INTERACTION:START:16800]
[CDI:ADV_NAME:Transform into a werebeast]
[CDI:INTERACTION:WEREBEAST_CHANGE]
[CDI:TARGET:A:SELF_ONLY]
[CDI:VERB:concentrate:concentrates:NA]
[CDI:TARGET_VERB:begin to change:begins to change]
[CDI:WAIT_PERIOD:1000] // This is the same as above, only this affects werecreatures that were bitten rather than being cursed by a god.
[INTERACTION:WEREBEAST_CHANGE]
[I_TARGET:A:CREATURE]
[IT_LOCATION:CONTEXT_CREATURE]
[IT_CANNOT_TARGET_IF_ALREADY_AFFECTED]
[I_EFFECT:ADD_SYNDROME]
[IE_TARGET:A]
[IE_IMMEDIATE]
[SYNDROME]
[CE_BODY_TRANSFORMATION:START:0:END:500] // These determine how long the at-will change lasts. START:0 means the change happens instantly, and END:500 means you'd have time to slaughter the population of a small village before reverting back to your default form.
[CE:CREATURE:WEREBEAST:DEFAULT]
Keep in mind, to make all werecreatures use this interaction, you'd have to turn off the default werecreatures in the advanced world gen. This would also make all werecreatures turn into your custom WEREBEAST creature, unless you made multiple copies of this interaction for different custom werecreatures.