Ok, made some tests, frankly I don't understand it anymore.
First renamed gmapprom1.img to gmapprom5.img, so that the Garmin doesn't recognize the House Number Search.
Turned on the Garmin, 16 symbols, instead of House Number Search the Address Search showed up.
Copied the MRT.gpi file to there, 16 symbols are showing but not the MRT in the Where to screen.
Made gpi file out of it called ZZZ.gpi.
In the Where to screen shows the ZZZ also with ur symbol. But no address search anymore.
Any possibility to get the "homemade" POI's into the Point of Interest?