I think its more a long the lines of fully coding such a thing into a game if its not there. which wont be happening for any game with no online mode.That's probably more along the lines of hacking right? If it's not in the game already
you would need to code in the online mode within the certain ROM of the game, set up a dedicated server for the game, and also distribute the ROM or the ROM patch. It would be extremely hard, but not impossible.Is it possible to create an online mode for the Inazuma Eleven or something 3ds game that I don't have?