'Sales Order' system. At stafe between quotation and invoice.

It would help with 'open order' or 'back order' system. As it stands, we can raise two customer documents – ‘quotation’ and ‘invoice’. We’re finding it difficult to process orders without an in-between stage. We receive many orders which take time to fulfil and it would be really handy to have a ‘Sales Order’ document. To take this further it would be good to have a 'document number' assigned to the quotation, sales order, invoice that remains the same throughout the process.