The mohaa sdk says fs_game - specify an alternate Game directory for add-ons
let's say you start a server with +set fs_game "mymod"
the game will load the main folder, and the "mymod" folder as a "secondary main"
So any modified file put there (like custom music, models, textures .urc files, etc..) will be loaded from that "mymod" folder without affecting the files in the main folder, keeping it clean.
and it's easy to switch between mods using game_restart <fs_game>