The backpack you have is designed without taking into consideration a hip belt. And a hip belt is what you need to take weight off your shoulders.

The role of the hip belt is to take most of the weight of the pack and put it right on your hips. That means that the pack has to extend that far down, and you have to have a belt that really settles around your hips and takes that weight off your shoulders.

If you add a belt to your pack, you'll certainly strap a belt around your mid-section, but it won't really take much of the weight off your shoulders.

Sorry, but you need a different backpack to really make this work.

My brother had a similar issue with his shoulder, and was able to achieve some success with a lightweight backpacking pack--complete with hip belt.