At the present time customers are unable to alter the statement template. Our products are evolving constantly evolving and to assist us meeting your requirements, and hopefully get them implemented in the future, we encourage you to leave feedback from within the product.
If this is UK Quickbooks online, you can achieve order numbers on statements as this is what I use as well.
You simply have to type the text "Order No: 12345" in the box marked "Message appears on statements" on the invoice entry form (see screenshot) obviously replacing 12345 with your order number. This will appear on each invoice line on your statements.
hope that helps,