Well, this is a little more complicated. What's normal map - it's derived height map, it's calculated F.e. from displaced plane (displacement value is defined by height map). What do we need to "solve"? We need to integrate normal map. I found a way, how to solve this. You go pixel by pixel in X or Y direction and add a "height step" based on normal direction in normal map. So if normal map is just blue, height map will be whole white. If it'd be violet, height map will be from black to white from left to right, etc. I could write converter, if you want (or don't know, how to create it, because it's really not-so-easy application), but it'll take some time or you could download one of existing converters - there's one in Photoshop i think.