This is for SR:TT though, but I think this also applies on SRIV.Team stuff is unfortunately pretty hard coded from what I've seen. I'm not sure how easy it will be to alter them and add more without being able to recompile the code. They're a packed enum for coop reasons. The best I could see is to repurpose the existing gangs for your mod, but I don't know if that fits with your vision.
Quote from Knobby when I asked about it:
This is for SR:TT though, but I think this also applies on SRIV.