Pre-assigned Invoice numbers in Sales Order Processing?

I'm trying to work out if this or how this can be accomplished in and during sales order processing.

The invoice numbers are generated externally - so we do NOT want to use NN from JDE.

I know I can prevent the Invoice print from assigning the invoice number, but where can I add this number to the sales lines that are being invoiced?  This functionality appears more for proof/acknowledgmenet purposes.

In A/R - we can pre-enter the invoice number when creating the invoice so no drama here.  Only when looking at generating the invoice as part of the Sales Order Process are we stuck.