I have not yet encountered an algorithm that can do what you describe: Take lighting information and turn it into height or depth information (from the way you describe it, I assume you're working from some kind of shaded relief original map). It's actually something that I looked for pretty hard about a year ago, when I was investigating new ways of converting movies to stereo 3d. Intuitively, it seems like something that shouldn't be too difficult, but I suspect that accounting for non-planar shadow-catching surfaces makes it a great deal more complicated.

I think if I were approaching this task, I'd use the original map as a template and paint the heightmap over it. Gimp has a nice gradient feature, where if you draw a selection, it will create a gradient from the center out to the edge. That would probably be a good tool to start with. After painting it up, I'd then run it through Wilbur, which is a free utility that can perform erosion simulation.