Until someone figures out the way the playlist and music is being stored on the HDD, this probably won't be possible.
I dug into it, and gave up myself. Although I was trying to figure out how to manually copy music over with a PC, without the need to install the disc ito the Z and wait for it to rip... But what I did notice, was that they are updating several files that are spread across several directories in the MSV partition. ou can put the music files where they are suppose to go, but they won't show up on the Z.... Well, unless you can figure out how to update the playlist files correctly...
Best and easiest(costly) fix, IMO, would be to buy the IPod adapter and an Ipod. This way you could make a single playlist, and put as many tracks as you want in a single album/playlist. Although the Ipod interface is borderline el'sucko... 