Is there a way to increase the number of starmap and battle music selection, the game can cycle through?

Right now I think I have three on the star map and one still for the battle screen.

All the music stuff sits in \Lua state\GUI\music\

It looks like it's not quite as simple as just dropping in new files though.
@Music.lua is where all the tracks are defined, and it's looking for track lengths, fade points, loop points etc.
If you have all that data for a new track then (in theory) you should be able to slot that into the appropriate places in the lua.

Looking through that file it seems like there were a few more tracks planned, as well as mutiple battle tracks, none of which made it into <current version>.

I think the answer is "yes, but it's non-trivial".