I don't think it's possible to ally neutral hostile to anyone... -.-. Anyway, I think what DoTA does is they have 2 extra non-player teams (I say I think because I don't play DoTA, I looked at it once and vomitted. However, I know how to accomplish what you are looking for). The extra non-player teams go on one side each. Too easy.
Although I do this to help people, it is nice to recieve Rep as a form of thanks or just to say that I helped you out, you know... it's sort of like a pat on the back. Please keep that in mind.