Referencing existing Address in ImpEx.

I am creating an ImpEx for PaymentInfo and it has a billingAddress field of type Address. But the AddressModel does not have a "code" field for me to reference it in the ImpEx (like billingAddress(code)).

How should I do it?

INSERT_UPDATE PaymentInfo; code[unique = true]; billingAddress(WHAT_HERE?)

Thanks in advance.