creating parallax maps in ZBrush?

Hi,

Does anybody know of a way to export a height map for the parallax shaders in unity from ZBrush? Or perhaps how to generate them from a normal/displacement map? I was wondering since unity doesn't seem to support the displacement maps I make in ZBrush.

Anyway, if there is some sort of a tutorial on how to do this, a program that generates a height map from a displacement or normal map, or perhaps an alternative way of getting a bit more detail from ZBrush into unity, I'd be grateful for any info on it! :D