Hi,
are you sure you want to use a File Library on a StoreOnce Appliance? I've given up on NFS ever getting stable (it performed well, though, until it broke). I've never got CIFS to acceptable throughput. I finally accepted what everybody told me before: the most reliable mode to use a D2D/SO Appliance is VTL mode. Even though doing VTL via iSCSI feels weird, it doesn't scale as far as a File Library, and proper purging of expired media is more of an art. With 8.x what you actually want to do is probably use StoreOnce Catalyst, but I entirely lack any stability experience with that. I just know it reads great on paper, but so did File Library on NFS...
As for the best way to copy that stuff, it's a really good question. One idea would be to create a VTL on the Appliance, attach it to the 5.5 cell through some media agent, copy over all the relevant sessions to VTL media (which behaves like emulated tape as much as possible and thus is more mobile than is a File Library), than detach the VTL from 5.5, attach it to the 8.1x cell, import all the virtual tapes and then copy it over to the real solution should that be something other than VTL (like when using Catalyst), When the VTL is no longer needed, it is easily removed. Should you stay with VTL, you are essentially done after importing.
HTH,
Andre.