add an old device

There is no method for the iPod nano to be added as a trusted device onto an Apple ID. It can only have media synced onto it or be used as an FM Radio device. You could still purchase media through iTunes, though it will require different hardware.

You do not need to add a device. Instead, simply buy the music from the iTunes Store, download it to your iTunes Library (on your computer) and then use Sync to add the purchased music to your nano.