Emily Tao has another pattern drafting tutorial in her library. In this video, she shows how to add a yoke to the back of shirt, with a dart underneath to help shape it to your back shoulder. The small dart comes out of the armhole, so she shows how to add that length back in at another point so your armhole retains the same circumference. Go to her video to see how to do it.
[photo from Emily Tao on YouTube]