Yes, you can do that. Hair Density and Length are UV-mapped shaders (2D) so you can paint a greyscale texture (white = 100%, black = 0%) and use it in those shader channels. You can also use pattern and noise shaders to create designs and avant garde textures.
When you create the hair, you have the option of using UV Maps or Geometry to determine aspects of the hair model…. I am not sure how far this choice translates into the hair shader, but it might effect you. If you are using a DAZ character everything is UV mapped already so there will be no problems.