A hidden field can be added, but not to specify process type. Instead, create a routing rule to change items from Borrowing to Doc Del.
1. Add a hidden field on the request form such as ItemInfo1 ='Doc Del."
2. Set up the routing rule so that if Iteminfo1='Doc Del,'
3. Route the request to Awaiting Document Delivery Processing in the process type "Doc Del"
See Creating Custom Request Forms for more information.