Hi LifeIsLike,
Thanks for posting your question here. We are aware that eBay has recently added "UPC" and "MPN" fields for certain categories and, while we have not been able to update our system to support those fields yet, we do plan on making those fields available through Auctiva with a future site update.
I am not sure when we will be able to add support for those fields at this time but, if you would like to be notified when we do, please contact our customer support team by mousing over the "Help" tab and selecting the "File Support Case" option and we will be happy to do that for you.
-Mike