Adding onto Lakshmipathi's answer-
In your Z report tell your abaper to
- pass the document number (VBELN) along with the Item number (POSNR) into VBKD and obtain the corresponding PO Number (BSTKD) for that line item.
- In your case pass the order number = VBELN= XXX along with item number- POSNR = 20 into VBKD and this will fetch the value ITEM PO1
At the header level obviously the item number will be blank, Just write this logic in a functional spec document and give it to your ABAPer, he should easily be able to make the change
Thanks,
Ravin