Enter zero as the opening balance in the customer set up screen. Enter all the open invoices as Invoice transactions. Use an Item linked to Opening Balance Equity, otherwise the P&L report will show incorrect income for last year. Use the last year's ending date (or the day before your start date) as the invoice date and actual due date so aging will be right. Make sure Opening Balance Equity is zero after all the other opening balances are entered
To clarify, are you looking to include a description of the transaction for the opening balance?