Can somebody tell me, if and if yes how, I can import the gpx file (or any other file) for campingsites from this website http://www.archiescampings.eu/dui1 for Navigator?
you can import your POIs using this PC tool: download.mapfactor.com/digger_12.zip once imported, copy .mca file where all the others are, most likely in android/data/com.mapfactor.navigator/files/navigator/data
Not if the data source is not free to be distributed for everybody, as is the case here. And it is not worth it anyway, I think. There is very limited data only, much less as I found for the same sites within Navigator's OSM data.