Essentially, the purpose I'm going for here is to have different versions of the musics, but still the same musics themself. Entirely new tracks may be missing that point a bit; though if they're in OGG format, or in IT or a similar format (including MOD), they can absolutely be used in a Flexi pack. If they're mostly the same with a few different tracks (such as the Master System version), that isn't such a huge issue; but if they're entirely different... :/
Of course, if you have the tracks in a suitable format, there's nothing to stop you creating a music pack yourself. Basically, use the Flexi toolkit, follow the section on creating a RES file of the musics, and the ARC file is your music pack; just rename it to the appropriate filename. For NeoCustLemmix, there should be a total of 23 numbered tracks (the 17 orig + the 6 OhNo), plus a gimmick track and a frenzy track. In all the music packs so far apart from the Acorn one (and I'm strongly considering changing this), the same Gimmick and Frenzy musics are used, which are the LPII ones. If using OGGs, note that NeoLemmix supports (but does not require) the "LOOPSTART" and "LOOPLENGTH" metadata tags.