In that case you can create a PO with the plant as a dummy material code (PO of 1 Unit). You can have a statistical GR for this after purchase is complete and then do an Invoice Verification to pay the vendor.
Thereafter scrap this material (551 Movt Type) in the Dummy Material code from stock and bring in the scrap (iron, steel etc) through movement type 552.
You can also move with an account assigned Text PO for the plant and go upto IV. This way you will not have to do the scrapping. Thereafter, bring in the scrap / material using any of 501, 521, 552 etc.