A lot of forums do this for RPG purposes and it's not that hard really. Make a new user profilefield and either:
- Make a new style (for that forum) and edit the desired templates, replacing the username variable by the profilefield variable; or
- Use forum id conditionals in your current templateset and use them to replace the username variable by the profilefield variable if in a certain forum.
This goes the same for any other variable you'd like to add/change, such as an avatar.
Good luck!