In Project Settings under Corridor Computation Settings you can select Honor Sign of Tie Slope - this allows you to enter a Positive or Negative value for a Cut or Fill Slope - note however that if you change this "Mid Project" that you have to go through all Slope elements and change the signs to make this work. When you have this value set you can define a Negative Cut Slope (downward cut) to take off the top of the hill - when you apply this you would need to do a Cut Slope to intercept the Existing once, and then maybe project 1' inside the hill and then do another Cut Slope to intercept the Existing again to take the hilltop off.
Alan