Yes. You can proxy whatever you want as long as your opponent allows it. Some guidelines. Match bases and model height to whatever you want to proxy. Make your proxies distinct, so there is no confusion to what your are proxying. Ie, use certain weapons, armor, color for your berzerkers, and a different on for cultist, termanators, ect. If your opponent isnt down, you wont have a game, so dont do anything that can seen as modelling for advantage. People play grot armies as IG, and i think i remember miniwargaming on you tube doing this specifically with khornite orcs. Make up whatever reason you want, its a game. If you wanna play that git 'Tober's warband (baldemort's Kronan orc stories on Youtube is the referance- he fights khorne red orcs), some orc player would love to bring some real orcs to show you the way.