If zombies kill all the humans is it game over? That could be problematic. If so, perhaps you could find a way to keep deathmatch mode on, only for that map, and.. something.
or you could use ZPA mode, so we all keep respawning, and teams autobalance. Then you could possibly put one of those objective Icons on the ball that shows you where it is, and putting it in the goal scores one point. The ZPA timer might be the only problem, but I'm sure that's fixable.
OTHER THAN THAT, I REALLY WANT SOME SOCCER. When it's finished post a link and I will TRY to upload it maybe. CP doesn't let me upload maps unless it likes them, and it's very picky..