Lemmixes are a lot harder to make than Lemminis, because everything's compiled into the executable, and the avialable source code is incomplete. Even in a complete state, adding custom data to it isn't the easiest, and that's assuming you use the same structure as one of the originals (IE: 30 levels each across 4 difficulties with the same oddtable positions, or 20 levels each across 5 difficulties with no oddtable, with the same music rotation although you can change the musics themself), if you are, it gets even harder.