Add this function in a subclass to get a notification just before the texture importer is run.
// Automatically convert any texture file with "_bumpmap" // in its file name into a normal map. class MyTexturePostprocessor extends AssetPostprocessor { function OnPreprocessTexture () { if (assetPath.Contains("_bumpmap")) { var textureImporter : TextureImporter = assetImporter; textureImporter.convertToNormalmap = true; } } }