I didn't mod Warband for years, but as I remember, Warband supports diffuse (which is obvious), specular maps and normal maps.
You can use substance painter to create your diffuse, create roughness, ambient occlusion and bake normal maps. Then you can invert roughness map (specular is basically inverted roughness - correct me if Im wrong), paste AO on diffuse map with overlay modifier and here you go. You've got your model textured in substance painter and ready for warband.
Sorry for my englando Kappa