Just rename the other tracks GMGEO2, GMGEO3, GMGEO4, etc.
The game will pick one randomly.
And it is possible to use for example GMTACTIC99?
Still, a folder would be better because you can copy to it your various non-UFO tracks
And it would be a big wish that the game would have an option or reading the tag fields and displaying them for a selected amount of time [so that the player would know what was randomly choosen]; or at least the name of the files [which could be re-named by the player accordingly like for example >>X-COM II Geoscape #6<<]. By I know that is a long shot because such display would have to be available on many screens [base, geo, ufopedia etc.]