Well technicaly, NPCs from singleplayer can appear on maps but theyÂ´re not active. they donÂ´t move. You can kill them but thatÂ´s like all. Also i just remember that Youda and some other guy (donÂ´t remember who exactly now, Hip maybe ? :)) were working on a gamemode on Mesa with active aliens (they could kill you, you could kill them), though my knowledge isnÂ´t very extensive in this matter, since im more of an IA person.