Edit: Misread your post, don't know about a normal map storing shadows, maybe having a look at the blue channel of the normal map? You can do it the other way around if thats any help; You can store AO in a normal map, Crytek's Polybump application can do this, if im not mistaken. "Horizon data (5): If you calculated a…
Ambient Occlusion Map: EarthQuake's Baking Method details some good ways to create ao for the diffuse map, You can also take the AO and bake it back into the normalmap... http://wiki.polycount.net/Normal_Map#Renormalizing An ambient occlusion pass can be multiplied onto the blue channel of a tangent-space normal map,…