No easy/native way to get this done in the standard NetSuite transaction screens. It'd probably involve a customization that consists of a "mapping" custom record (UPC to Item), a custom line level field to hold that custom record, and then a client script to populate the associated item in the native field.
If you want to help them find them items on the transaction, and do not have access to a developer, then you could use something like
SuiteAnswer 78744.
Alternatively, and seemingly not what you are looking for, if you simply want to global search by UPC then you should be able to do that by using a custom item field and enabling the "global search" attribute for that field.