There is a setting in the configuration menu that will enable you to edit
closed purchase orders. You will then be able to add the supplier.
If you would still like to delete it, you could add another item to the PO
and save it to make it a Partial, then receive them all to negative
quantities (1 extra negative on one of the closed items, and 1 positive for
the new item, so it will enable comitting. After comitting there will be 2
items that are off. Do 1 positive on the one you did an extra negative on,
and 1 negative on the new item.) so the PO is open, then delete it.
I'm sure I made it sound confusing, but it is very easy really.