Karen,
You could create an “auto-code” customization using functions that will generate the next-sequence UPC code using your company GS1 prefix and the UD table you store those values in. We did something similar for generating new part #'s that needed to follow a specific naming convention based on material type.
There are a bunch of threads on this forum that discuss auto-numbering - this thread is a long one about auto-numbering for customers) but has lots of good information that I think would be relevant for you.