Within reason, I don't see why you couldn't use the morph tool. Zbrush would be an excellent choice since its designed to handle that sort of workflow.

Even though the edge flow is basically humanoid, you're going to run into problems especially at the ear. Avatar ears are set much higher on the head than Mil4's, so the edges surrounding them might become "scrunched". Not a good thing as far as texturing / displacement is concerned.

Eye sockets / nose bridge will be tricky but not impossible.