Where to associate UPC with the item in PIM? UPC is 12 digits and cannot use GTIN being 14 digits

Summary

Where to associate UPC with the item in PIM? UPC is 12 digits and cannot use GTIN being 14 digits

Content

FG products (saleable) needs to be identified and associated with Universal Product Code (UPC) which are 12 digits (11+ check digit). 
In the PIM, under Relationships tab - there is a provision to specify GTIN which much must be 14 digits (13 + check digit). 
UPC-12 can be converted into GTIN by adding two leading zeros; but that does not help when scanning the UPC from product packaging as it is not recognized - the scanned value has 12 digits whereas the system (in PIM) has 14 digits.