I have been looking for information about this topic for a while from forums etc. since I want to assign a few of my own sims to NPC roles, butler for exsample. For most of the roles it seems possible. A sertain role might need sertain skills + hidden traits, usually two of them (NPC career and NPC trait). The traits can be added with MCCC or with codes.
I haven't tried it yet, but I'm planning to make a sim (or download from the Gallery), give them the skills needed, and adding hidden traits with MCCC. Then marking them as unplayed (and not in the world). And finally getting rid of the current NPC sim. The game should pick my new sim for the role now.
Edit/ If anyone with more experience knows better, please correct me :)
