Assign a Sales Order to existing Purchase Order, rather than create a new one

Assign a Sales Order to existing Purchase Order, rather than create a new one

I understand that the "normal" chain of events is to create a sales order and then use that sales order to create a new P.O.  But, it would be nice if, in addition to that, one could assign a sales order to an already existing PO.  It would be convenient as I sometimes create POs first....
-Tom