No, you can't I fear. Mechanics and basic engine systems are likely not changable with the editor (outcome of the AMA). And you would need a sort of matchmaking system and a system which would allow real people to play against each other.
Would be mucht better to have a PvP mode from the beginning (or in a patch/update later?). As I said, it's also a strong marketing and sales argument because it would be a complete new game mode (without THAT big effort of Larian I guess).
