Is it possible to add UPC codes to items, that can be used to scan during inventory counts?

Summary:

We have a customer requirement to be able to scan UPC codes during inventory counts that link back to internal item numbers.

We are aware of using the GTIN number, but this would require adding digits to barcodes and re-barcoding a large sum of items due to requiring 14 digits. GTIN also does not solve for when standard barcodes that come on packaging is used (EX: Barcode that is printed on a bottle of water)

Is there a way to either use cross-references, a DFF, or another solution to link UPCs to an item for scanning?

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