I don't believe there is any way to export an audio track with more than a stereo pair from FCP; at least not in a single export. You can, however, turn your audio tracks on and off selectively and export all tracks to separate files, either as mono tracks (1 at a time), or stereo tracks (in pairs). The safest, most portable format that is lossless is AIFF. Simply use Export with Quicktime conversion and select AIFF as the output format. Pick 48kHz, 16bit stereo and export one pair of tracks at a time.
You can reassemble the tracks in a Quicktime file (the MOV format supports as many audio tracks as you want), but a professional audio service should have no problem with multiple AIFF files, as long as they all synced to the same start and duration, which they will be, if you export them all from the same sequence.
You may want to visit Apple's FCP discussion group for more ideas.