When you input an opening balance at the point a new client record is created, all Quick File does here is generate an invoice matching the opening balance supplied with the name “[Client name] Opening balance”. You can actually do the same even after a client has been created, just generate a new invoice matching the debtor amount and any future payment relating to older invoices can be attributed to this initial debtor invoice.
An alternative of course is to just enter any outstanding invoices in full for the client. The aggregated method is just a bit quicker if there many clients with multiple unpaid invoices.