Add a custom field that auto populates the PO number unless otherwise specified by user

In some cases there will be an "Invoice Number" field on a purchase Order that the user will have to enter. At the time when the report is generated to import into a different system the Invoice Number field might not have been filled in. In that case the PO number needs to be entered into that field.
At the moment this needs to be done manually by the user.
This is a request to add a custom field that will automatically populate with the PO number but, it has to be editable to allow the user to enter the invoice number at a later stage.