FMIM files are WMA files, with track info strapped on at the beginning. They are not encrypted / there is nothing special about them. If you want to take them from the 360 harddrive to the computer, you just have to delete the FMIM header, and add a .wma extension.
On the other hand, to put WMAs or MP3s on the 360 harddrive directly, you would need to first make sure it is in WMA format (MP3s would need to be reencoded), then add then FMIM header, then add an entry to mindex.xmi. Unfortunately, I don't know of any research offhand into the formats for either of these, so it's the recreation of the FMIM header and the editing of the mindex (not signed) that would prevent you from editing it for now. If you or someone you know wants to take this on as a project, good luck!
It shouldn't be too hard from the looks of it.
roofus