Not much of my friends play HALO anymore but I still do. Is there a hack that makes AI into multiplayer, so I can play against the AI in multiplayer maps?

no

It's possible, I've seen a mod for PC halo blood gulch with a bunch of Elites standing around for target practice, etc.

Yes, there are actually a few of them. there are ones that use invisible warthogs to spawn and there are others but i forgot how they spawned. The ones that used the warthog would eventually freeze the box up (too many warthogs). There is a way better one without the warthogs and the AI is much smarter. If your actually interested in it Pm me and ill give the mod to you or tell you where its at if you already havnt found it.

Edit: Those were on xbox if you didnt realize, had to make sure i said that for the special people.

a friend of mine with a modded box has it so some of his guns shoot warthogs. He said that he can get in them, drive them around, fullly functional warthogs, but too many freeze it up. So wouldnt you be able to shoot some elites

Yes, you can shoot the elites, but they have no AI(artificial intellegence). They just sit there and look cool.