Shopperpress SKU Import

I can’t get the values for “SKU” to import into Shopperpress. I have an import file that has a column heading of UPC. I’ve changed that to SKU and the values don’t import. Should I just add my UPC as a custom field then? I’m using a third party csv importer I think it’s called Ultimate CSV Importer. And it’s nice because it provides the option to map my csv headings to database keys. But I don’t see a key for SKU.