Add Custom Sale in POS

[SouthernBoston]

In my brick and mortar store I manage several vendors. Most of them we do not want to have inventory loaded onto Shopify, so when their items are purchased by a customer, I want to be able to still have Shopify track the sale to that vendor for reporting purposes. If I have them setup as vendors already in Shopify, is there a way to add the vendor field to the "Add custom sale" button I use to make the sale on my POS system? If I could add that field I would be able to track each vendor's sales. Thanks!