How to "jailbreak" or add songs to my ipod nano without deleting whats already...

...there from the previous owner? I bought a 4th gen ipod nano off of craigslist.com and really like the music that is on there, all of which I don't have. There's 11 gigs of free space that I want to put my music on, but I really don't want to lose the awesome music that was already there! Does anyone know how to make this thing work like a normal mp3 or usb type thing and be able to put my music on without losing the songs!?!?
 
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