It would probably be quite doable, but in the strictest sense, it might need at least some modification in order to work entirely correctly with more. There may very well be things that only make sense with exactly one or exactly two players.

Halo 1 may be able to handle a third weapon if you trick the game into giving you the PoA pistol when you already have two guns, but not everything in all software in the world is implemented as magical linked lists and other expandable structures.