I actually know quite a bit about adding a new character slot, i was researching it for like 2 months. Your better off doing it via code.
If you mean replacing an existing one, thats way too easy and being done. Model hacks and animation hacks are coming, PSA will be complete soon, there is already a major project happening.
No I'm not replacing characters already in the game, I'm putting new characters in that are not in anyway related to the playable characters right now.