I don't know anything about modding but what other teams are you planning to add?
I am not sure if there are other animes that are basketball-themed, but can you make 2 teams (Kanagawa and non-Kanagawa teams, both from Slam Dunk series) and their uniforms are like shoyo-ryonan (each player's jersey is their school jersey? Oh, and if possible, retain their school jersey numbers too (Akagi, fujima and maki all wear number 4 on the same team)
Here are my sample lineups: (I can submit later the full 15 man rosters if you want)
Starting lineups:
Kanagawa:
maki
fujima
rukawa
sendoh
akagi
Non-Kanagawa:
Morishige
Kawata
Eiji
Aiwa no.4
Sannoh no.4
edit:
about adding new teams not from SD or KNB, I read from other sites about Dear Boys (but I am not familiar with that series). I am not sure if many people know that series, so it might be good to get some response from people first about that since it's a lot of work.
I think the kanagawa and non-kanagawa teams are easier to do because the player files are already there. I don't know how to do that though, so yeah I was just suggesting. Good luck with your plans.

edit2:
I think you can also apply the idea i mentioned above (kanagawa and non-kanagawa SD teams) to KNB. like 1 team for the region where seirin and shutoku play and another team where the other teams play. Same concept for jersey numbers. It would be cool