Technically, it should be possible, but highly unlikely that your customer will be happy with it. MOD files were created as sort of an upgrade from General Midi, with actual audio samples, but still small enough to be used in games. If you convert an entire MP3 file, it will become much bigger than the MP3 file itself, and probably way to big for the game it is intended for. Making MOD music is a skill on its own, just like making general midi music.