Product: Transaction Manager
It is possible to import an invoice or a batch of invoices if they have already been imported and even closed out in your business system.
When you click Import, search for the Invoice number (found in your business system).
Set the Transaction Status to All Txns.
Depending on your business system, you may need to add an additional setting to indicate that the invoice was already closed in your business system (i.e. in Sage you'd indicate the invoice was "posted").