Modding Genesis Games

Does anyone know how to mod (legally obtained) roms for Genesis and snes? I really would like to put Jordan in Showdown 94, NBA JAM and a couple of other old games.

It might be possible with a hex editor.

Yeah hex editor is the way to go. This guy made a nice NBA JAM hack called old-school.
http://www.youtube.com/watch?v=xxusez2uTmM

Awesome.