I'm not sure I understand what your looking for.
Cross shipping is standard function in RMA. if you select the cross ship check box, and you have replacement as your customer action then it creates a sales order when you generate the documents even though the item has not yet been returned by the customer.
if you don't check the cross ship and have replacement as your customer action then it creates the sales order after the RMA receipt entry is updated.
is that what your looking for?