I didn't know what you meant by PO, I associate PO with Post Office Boxes, that's why I suggested that. Why do you need the user to entere the PO number into the checkout form? There is an order ID for each of the orders.
That would be a good recommendation for the next release of the store. If you could post that in the wishlist forum, it'd be great.
Well, you can modify the form using Form Builder, but after modifying the form, you will have to add some additional changes to get the information stored and inserted into the database as well as sent out into the emails.