BFB and Imported Billing Numbers

 I have a client that wants to use balance forward billing.They generate sales orders and create invoices daily but send to the customer once per month as a consolidated invoice. They also create credit memos that are created via a sales order with negative quantities. For current month activities, they want invoice and credits for the current period to be part of the BFB invoice - no problem, that is standard.

The challenge is for cedit memos created in Sales Orders with negative quantities that are against invoices from prior periods. They want those seperate from the current month activities.