Well technicaly, NPCs from singleplayer can appear on maps but they´re not active. they don´t move. You can kill them but that´s like all. Also i just remember that Youda and some other guy (don´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 isn´t very extensive in this matter, since im more of an IA person.
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