I don't think there is something like that. You can (possibly) try private servers of that game you're talking about. There are no ROMs or such for MMORPGs. Your best bet would be cheating on the game.
I think you should mention which MMO you're looking to play privately. I know someone from /r/FinalFantasy has a private FFXI server running with increased experience, money, and item gains. They were trying to get the game a bit more in line with a single-player experience, as well as making the game more conducive to smaller parties of players.