Purchase requisition to Purchase order process

Hi all, 

we started to use the purchase requisition in ax recently.  Before this process was handled manually outside form ax.

Now the user creates a new purchase requisition ( POR0001) with two lines. Next, it goes to the workflow for approval. Once approved the procurement team release the purchase requisition to a purchase order (PR0001). The purchase order does have two PO lines.

The user wants to add another two lines to this purchase order (PR0001). Therefore he creates another purchase requisition (POR0002) and wants to add this POR0002 to the existing purchase order ( PR0001). 

Is there a way to do this? 

Thanks in advance!!