You can get SATA to USB adaptors which should allow you to copy the files onto the SSD drive using your PC or laptop.
The other option is to install the SSH in the Nucleus and connect to it over the network via SMB file sharing. You should then be able to do that even without a Roon account as it happens at the OS level, not within Roon itself.
He’ll still need to wait for Roon to scan the files once he logs in, but assuming his files are tagged well and the any multi-part albums are named correctly, it should be pretty much ready to go.