Dear Dave,
You can create infoset in SQ02. Please select ‘Direct Read of Table’ instead of logical database. Enter table BSEG (Yes, it is possible to use table BSEG in query through direct read of table) and save the infoset.
Go to transaction SQ01 (Make sure you have assigned infoset to user group), choose the infoset you have created. Now in BSEG you have fields for purchasing document number (BSEG-EBELN) and item number in purchasing document (BSEG-EBELP). Mark these fields for selection screen and mark Purchasing document number (BSEG-EBELN), item number BSEG-EBELP) and Document number BSEG-BELNR) for output list.
This way you will get the accounting document number for PO numbers.
Regards,
KSandeep