Firming planned orders by adding lines to an existing order in Microsoft Dynamics AX
For Purchase orders and Transfer orders, we have a very handy functionality in Dynamics AX's Master Planning that has been there since the beginning: While firming a planned order, add it as a line to an existing order.
Add to existing Purchase orders
The surprising news is that for planned purchase orders, I can select from a list of existing Purchase orders for any vendor and in any status.
The field "Purchase order number" allows selection of an existing Purchase order to ‘firm to'.