If you place an order outside of Backbar, you can and should still enter the order into Backbar.
If you place an order outside of Backbar, for example by telling a sales rep in person or on the phone, enter the order in Backbar the same way you would normally. However when you are on the shopping cart page, uncheck the checkbox next to the vendor contact names, so that the order won't be sent to them. Click the Place All Orders button and the order will be generated within Backbar, but not sent. Be sure to still close the order by clicking Confirm Order Received so the products get added to your inventory.
This method can also be used if you receive additional products that weren't on the original order. Simply create a new order, don't send it, then confirm received.