No, that will not work. That is for pmnt advices.
if solution with special field doesn't work. You can have PO number in vendor line item on document just if you will make this field available in field status ( document/GLA ) and if you will prepare substitution.
Otherwise I saw customer solution by copy FLB1N program and extended it for customer code where PO was entered into item posted to vendor form GR/IR item from the same document.
Jan