The first time I started RoR I was wondering if when the game was new it didn't have normal maps?! I couldn't remember, been a long time. I'm new to RoR btw. awesome project.
I was curious about the normal maps and found this topic.
Gave it a try to see how well you can create a normal map form the actual diffuse maps.
This is NOT a method you would use on an up to date asset, but probably the only thing you could do in this case.
Since the game was not supposed to have normal maps, they heavily added lighting information to the diffuse map which diminishes the actual effect of a normal map if added afterwards.
See the result of my test below.
Considering the amount of assets and work needed to create normal maps and the results you can get, I don't think it's feasible.