Are the UPCs created by the system re-usable for other purposes like physical CDs fulfillment?

YES.

The UPCs can be used for physical CDs.

To generate the physical barcode, you can use http://www.barcoding.com/resources/barcode-generator/ and fill in with the UPC assigned by the system on your Release and the barcode will be generated.

Please use the below settings:

Barcode Data: insert your UPC code here
Barcode symbology: select EAN/JAN 13
Output Format: select the image format you prefer (.png, .jpeg, .gif, etc.)