Sorry, I've been distracted by getting shadow mapping working in the next game.
Well that just increased the odds, so it is very possible you still don't see it very often.
You could do something like this:
NpcFreeRecruitPriestMale1TherapyMod addsTo NpcFreeRecruitPriestMale1
and do the same thing for NpcFreeRecruitPriestMale2, NpcFreeRecruitPriestMale3, NpcFreeRecruitPriestFemale1, NpcFreeRecruitPriestFemale2, and NpcFreeRecruitPriestFemale3. That should make all priests have the therapy skill.
Files in assets004.zip override files in assets001.zip (nothing ever gets deleted from the original files).
Personalities are in Database/Monsters/npcPersonality.gdb
Off hand I forget how effective the therapy skill is. There are many ways to try to deal with insanity though: R&R does work fairly well, insanity items, full rations, don't let others die/go insane/or leave, and make sure to cure things that are affecting your clan members.