You'd have to manually change a lot of stuff. And I mean a lot... It's probably possible though. Actually, come to think of it.. let's see, what would have to be special about having an additional team...
- Unique IFF Indication
- Ability to select that team
- Scoring system for that team
- spawnpoints for that team
The first and third are easy to implement, though I don't know how you would do the second or fourth. The second might be as simple as a GUI or as complex as an engine change. I get the feeling though that the solution to the fourth one is going to be rather hackish unless you actually can integrate additional teams which as I said before, idk if it's possible.