You can kind-of convert a normal-map back into a height-map. The normal map defines slopes per pixel. If you step from one pixel to the next treating all the slopes as lines (in 1d) or planes (in 2d), and fitting them all so they connect to their neighbours, then you end up with a surface that you can calculate the height of per pixel. Depending on where you start this process from, the whole surface will be shifted up or down, so you'll want to renormalize the heights, by offsetting them all so the average height is 0, or 127 or whatever.
If a 2D normal map isn't based on a physical model, or simply due to quantization errors in the normal map, this surface might be impossible, like an Escher-esque staircase. I guess you'd want to apply neighbour constraints in a few iterations, in different orders to converge towards the most sensible solution, which is obviously going to be slow...
IIRC, the nVidia photoshop plugin has a filter that can do this for you, so you could ask the artists to generate them that way.
Or, you could implement this algorithm yourself and either execute it on your loading screen, as each normal map is loaded from disk, or make it into an executable that you can use to batch-process all of the normal map files ahead of time.
Edited by Hodgman, 14 October 2013 - 01:23 AM.