The RC 20XL will allow you to load anything you want into it via the mini stereo jack.
It comes with a sample CD and some awesome backing tracks already loaded into it. So yes you can put whatever you want into it. Want to jam over a Martin Luther King Jr. speech? Just load it in. Got your favorite 8 bar (longer or shorter) groove with full band accompaniment. Just load it in. You can do it from the computer or a CD player or even an mp3 player.
When I took mine to out of town gigs I used my mp3 player to load stuff. Way versatile machine.
Going the other direction:
The XL does not have a SD card option. You would have to transfer the file into the computer as a WAV the old fashioned way, line it in and press record.
"No Tele For you." - The Tele Nazi
Ha! Tele-ish now inbound.