The game uses
for bump, and specular maps (grayscale images) for shininess.
If you don't want more than one texture to have the same normal/specular map or don't need to name the normal/specular maps anything specific, it should automatically use them if the normal map has the same name as the main (diffuse) texture, but with .normal before the file extension. Same with the specular map, but using .spec or .specular (I don't know which) instead.
Otherwise, or if the above doesn't work, put this line of code at the top of the mis file for every texture you want to have normal/specular mapping, changing the file path to whatever the file names are.
registerMaterialTextures("platinum/data/interiors/bgasdigbrg", "platinum/data/shaders/DefaultNormal.jpg", "platinum/data/interiors/gabhsdg.spec.png");
You don't need to put .normal or .spec/.specular in the file names for this. The first set of quotes has the diffuse texture path (it doesn't need a file extension), the second has the normal map, and the last is for the specular map. This example just uses a texture that vanilla MBP has for the normal map.