You can increase the poly-count of a body part by a number of means, but then the existing morphs for that body part won't work any more. That's not a good thing when you're working on the head.

If you're trying to create the sort of forehead ridges common to Star Trek aliens, you could spawn a new prop off the head, subdivide it to a higher resolution, morph the heck out of it, and then parent the prop onto the head as a virtual prosthetic appliance.

Displacement mapping is another option for P5 users.