It's entirely possible to mod the game to allow you to choose to be a werewolf or have whatever other interaction upon game start: you make new creature castes in a creature definition and give them an interaction that allows them to transform into a werewolf.
For instance, you take the human creature raws, and copy the castes: in vanilla there are only FEMALE and MALE. Make the duplicates of these castes something like FEMALE_[some-name-here] and a likewise ID change for males. Then just add an interaction to the new castes that allows them to transform into a werebeast, either at-will or on lunar cycles (although I use at will). You're going to need to know how to modify creatures and interactions to do that, but done successfully you should be able to choose to play as someone who can innately transform into a werewolf at will.
I did exactly this for my Spellcrafts mod, which
you can download here if you don't want to edit the raws yourself. If you don't want the other castes and interactions it adds like the casters you can go into creature_standard.txt and delete everything but the vanilla MALE and FEMALE castes and the male and female werewolf castes, then delete interaction_spellcrafts_secrets.txt. Then you probably won't encounter or have to deal with any sort of casters: just the new werebeast castes you can choose from in addition to "are you a boy or girl" when creating your adventurer. By default the werewolf you transform into isn't [CRAZED] like werebeasts generated by the game, because it gets pretty crazy when people randomly transform and then go berserk. Or everyone decides to hate you all of a sudden.
Oh, and if you download it and it doesn't work out of the box, let me know. Because that's important to me.