Linking PO's to invoices & vice versa

Would like the ability to link a PO to an invoice if it wasn't generated from the invoice first. Depending on the vendor, I send a PO to the vendor to get pricing prior to invoicing a client, but DF doesn't link in reverse and there's no option to link the PO to a new invoice and vice versa. Love the new feature of seeing invoices linked to POs, but this addition to manually add would be great. Thanks!