Coat the hens with about 1/4 of the mushroom soup.
Mix the remaining mushroom soup with mushrooms, olives, zucchini, and garlic salt in a bowl. Stuff the mixture inside the hens. Arrange hens, breast sides up, in the slow cooker. Spoon any remaining soup mixture over the hens.
Cook the hens in the slow cooker on High for 4 hours.
