Add custom types to an .img or .mp map

Hi, I’m trying to add new POI types into am Garmin map.
I edit my map in GPSMapedit, store nit in the .mp format and in the end i convert it back to .img.
I#ve tried a couple of ways to use new custom POIs via a type and bmp files but failed to add ist to the map. Can anyone help me please?

br Stefan