Whilst processing an order, it is possible to add a product to the order if the customer wishes to make an amendment to the order. You will be able to do this within the order edit page by selecting the order edit pencil.
At the top of the order edit page is the 'Add Product To Order' tab as shown below.
By clicking on this tab, a new window will pop up showing you the home page of your website. You will now need to go through and select the product(s) that the customer has requested. Once you have selected the product, you will have to select the 'Add to Order' button as shown below.
You will then be taken back to the order edit page where you will see the new product within the order. You must also note that you can not take payment through this method so alternative options such as payment over the phone would have to be arranged.