Purchase group allocation to Purchase requisition

purchase-requisition-quierytxt.jpg

Hi,my client bussiness process is first they will create purchase requisition.once purchase requisition is approved,in ME52N tcode,they will assign purchase group to purchase requisition.The problem is if there are 50 line items,we have to manually assign purchase group to each line item but they dont want to assign in each line item.for all the purchase requisition,purchase group should be assigned to purchase requisition in a single shot.same purchase group will be assigned to entire purchase requisition.could any one help me in resolving this issue.thankspurchase-req-default-values.jpg.

Suppose if i use default values of purchasing group,it is coming after the already created purchase requisition line items.

thanks