Adding GBs to Ipod nano 3rd generation 8GB?

iPods do not have expandable memory. You can't add any more capacity to one without completely replacing the drive and Apple don't provide such a service

"I've heard you can add GBs to your ipod."

Never heard of such a thing.

A nano would be difficult, since the storage flash memory is not in any standard format.

The full size iPods with a hard drive can be upgraded with a larger capacity hard drive of the same physical size (thickness) and the same type of connector. Generations 1 thru 4 use the same connector. 5th gen and classic use the newer type of connector.

Unfortunately, those drives are not cheap, and you can't go to the local computer mega-store and buy one.