I think this would be easy. Have you moved a file from the DP to a computer yet? Basically, you choose the track, assign it to the Audio Depot, connect the DP to your computer USB cable, set the DP to USB mode, and move the audio track (a WAV file) onto the computer. I was confused by the instruction manual, but I have found it easy to do. I did that several times today!
You could pass the SD card to the band. You could put the file on a thumb drive.
Theoretically, you could email it, though most email services will refuse big files. If the file is too big to email, you could transfer the large file with a Dropbox (or similar) account. You upload the file, send an email, and the recipient retrieves it via internet.
So quickly - band cuts tracks and makes a trial mix. They upload from their recorder to Dropbox. You download the mix file and import it into your DP. You record your killer vocal track!
You move the vocal track to your computer and upload to Dropbox. The band retrieves it, moves it to their recorder and mixes you in.