Could Sales Invoices that have been generated from a Sales Order/Quotes be amended so that any "additional" stock added at the invoice stage have their stock levels updated when posted?  

Presently, as the SO updates stock levels upon despatch (not when the invoice is posted) any "new" stock that is then added to that invoice does not have its stock level amended when that invoice is then posted. Quite often we raise an invoice from a sales order. So when our customer wants to add to the invoice it means that we either have to generate a separate invoice or manually make the stock adjustment in the product module - if not the stock levels are not accurate.