Add item w/o reference to PO in an inbound delivery

Hi,

Is it somehow possible to add an item, w/o reference to a PO, in an inbound delivery and have no errors at GR? E.g. Inbound delivery was created for a PO with 1 item. Now I want to add an additional (special) item directly in the inbound delivery. Technically this works but I get an error when posting GR.

Thanks