If you got two distinct forces (teams), one being the players and the other the creeps, you should be able to create a unit that is owned by the player representing the creeps force.
If the creeps are in the same team as players, you have to change the diplomatic behaviour of every human player to enemy of the IA player representing the creeps. This is easy to find in "Game" or "Player" trigger actions categories, I can't remember it without opening my WE.