Can I add the soup to my preorder pay cash for that and the rest on my dining plan?
They don't split checks on pre-orders. One payment method.
Honestly, the soup is a good use of a snack credit.
According to this poster on the following thread, BoG is no longer allowing a snack credit to be used for the soup....
https://www.disboards.com/threads/french-onion-soup-at-bog-no-longer-a-snack-credit.3702984/