Release firm planned order to Purchasing from ASCP 12.1.3

How to firm a planned order to purchasing from ASCP 12.1.3?
In the ASCP Workbench it is possible to firm planned orders for buy items and enter a different needby date then the planning suggests. However a purchase requisition does not have a firm flag, so it cannot be passed to the purchase order.

Business case:
During order acceptance (in ETO environment) the suggested due date is too late to provide an acceptable available to promis date to the customer. The buyer negociates with the supplier about an earlier delivery date. If the buyer and the supplier have reached a mutual consensus on the delivery date this negociated delivery date must also be set on the firm planned order for this item.  It is necessary to firm the planned order for this buy item and release it. However the requisition does not have a firm flag. During AutoCreate the purchase requisition is transformed into a purchase