You can use NPC-players, which would be someone who is considered a player but is just written out by you. Like you said though, they would not be allowed to be factored into combat or any other game mechanics.
Unless you are fighting said NPC player, in which they would be regarded in combat as a monster. However, unlike normal player killing, I don't believe this counts to turn you orange, but I'm not sure the exact rules on that.
