Hi Tarek,
In your point of view not possible in SAP. You can raise DPR / DP against PO with item number. You can't raise DPR / DP against PO number without line item.
You may create separate dummy material code ( Single code ) for all condition's & raise DPR / DP. No other other solutions are there. You can able restrict values with PO header level condition types. No other SAP notes also available.
Regards
Mani Kumar