If you had an external enclosure that allowed simultaneous USB / Ethernet access, then yes, it would be possible. You would connect your enclosure via USB to the DSM-520 and share your drive on the network. Manage your files via the network from your PC and play them on the DSM-520 from USB Direct.
Copying movies over ethernet is to slow for me. I use a small, cheap, $19.95 USB enclosure with a 250gb IDE HDD. Hook it up to my PC, copy my movies and then move it back over to my DSM-520. Super fast and easy.