I don't think you can add a backrest on the stock CVO seat. If I had to take a guess as to why, I would say HD wants more of your money for a new seat that will accommodate a backrest. More than likely you will have to add a Street Glide seat or go completely aftermarket find a seat that is set up for a backrest.
Ray, checkout my friend's site at www.cvobackrest.com, the owners name is George Assali & his products are top notch!!! I purchased my backrest for my 18 CVO FLHXSE & couldn't be happier!!! Tell him that Lt.Robert Steele from Pittsburgh referred you!!!