Wall climbing exercises often can aggravate impingement problems because the humerus is internally rotated when doing the exercise. This causes the greater tubercle of the humerus to press into the acromion process. It is better to have the humerus externally rotated when abducting the arm.
I found a simple stretch that helps to improve range of motion at the glenohumeral joint (shoulder) without aggravating any impingement problems. To do the stretch you need a stretching handle and stretching strap, which can be found at: http://simplefitnesssolutions.com/stretch.htm
With frozen shoulder there are often adhesions in the inferior part of the joint capsule. Pulling the humeral head down stretches this part of the joint capsule and can be helpful in improving range of motion.
Go slow! All stretches should be pain free.
Do not swing on the shoulder strap creating momentum