Here is a discussion on how much force is required to fracture the skull
How much force does it take to break a human skull?
Here is a discussion on how much force is required to crack a coconut
How much force is required to break a coconut
Human Skull=1100 lbs Coconut=1400 lbs So, seeing legitimate coconut breaks, theoretically you could train the hand to crack a skull
That being said, the head is very mobile and still very hard. While it COULD be done, it would more than likely be a rare occurrence.
The human skull varies widely in thickness, shape, etc, depending on the exact portion of the skull under discussion. Coconuts are relatively symmetrical and roughly the same thickness (except for the "eye"). While the figure of 1100 lbs of force may be true in some areas, it will be waaayyyy off in others.