Unfortunately, it is not possible to import old invoices created in different software. It is only currently possible to import CSV lists of customers, suppliers, and products.
However, you can recreate an old invoice on the software. Create a new invoice on Debitoor and enter the exact same information found on the original invoice (including the invoice number and date). If the invoice has been paid, you can click "Add Payment" and enter the amount paid and the date the payment was received.