If you have ISRCs from outside eMastered, you can use them when uploading new music.
When the track section asks if you've released this before, click "Yes - this exact audio was released before."
With this option selected it will show you the ISRC field where you can enter your ISRC code.
Note that custom UPCs from outside eMastered are not supported. Each eMastered release gets a unique UPC generated internally. If you need a UPC for physical use or elsewhere, contact your local GS1 office.