Forcibly pronating your hand when your doing a tricep exercise is supposed to target the outer portion of the muscle. You might be able to add some width by concentrating on the outer portion if you don't already. Cable tricep push-downs with a rope are good for this.
I've read that doing dips with your palms facing outward is supposed to help with the outer portion of the muscle as well. However, this one sounds very painful and awkward. Can't imagine it would be good on the shoulders