You can add it without replacing FS2 music soundtracks.
Simply open the music.tbl(you've to extract it using VP View first, you know)and open it with Notepad...copy and paste one entry(for example Genesis) somewhere in the table, and change the filename of the specific tracks you want to add(for example, Battle 1).
If the soundtrack is long and variable, you may create a completely new music.tbl entry.
Requiem for a Death is an example: the creator has separated different parts of the music file and created btl1, btl2, btl3, enemy arv, allied arv, death and goal sounds that work ingame.