Ability to enter purchase order which transfers to invoice

Membership Pro - As we deal with governments with financial audit requirements they will often provide a Purchase Order which needs to be included on the invoice for payment. 
As the invoice is generated automatically, we can't enter the PO ourselves - however the PDF is editable so the customer can enter the PO themselves.
It would be great for customers to be able to enter their PO when renewing membership and for that to transfer to the invoice as generated.

I don't really understand this question. What if you create a custom field to allow entering that PO number?

I can imagine that you create a custom field that will be imported in the invoice.
I have not much experience in Membership Pro but in Events Booking you could create a custom field in the checkout that will add information in the invoice. Maybe Membership pro can have that mapped/synced that to that custom field?