I used an entity named 'ai_relationship', as far as I know, you can only add it in the map editor (hammer). I set the "subject", in this case npc_combine_s, to "Like" the !player entity.

I think there are some addons on the workshop that let you toggle being on the Combine team for every map, though I don't know their names since I am no longer subscribed to them. I think it had something to do with "NPC Vests"?

Yes, I know that mod. I want something that doesn't have the screen effects and doesn't change my weapons when it puts me on the combine's side.