If client has a barcode scanner, there is a setting that will allow them to add products that do not exist in the Management Console or the current menu directly from their POS system.
In the Management Console go to Settings > Retail Settings, check "Create item on unmatched barcode."
When a barcode is scanned and does not match any of the products, the client will receive an "Add Extra Item" pop-up.
Fill in the appropriate fields and save it as a new product.
It will automatically add to the order and be saved to the Management Console for future use.