After some more time of investigation and this little sentence:
Every EM must contain a Natural technique for default playback (no Actions needed)
which I found here (sorry for not finding it before posting):
, I could manage to achieve what I wanted.
I now understood, that dorico always sets “Natural” for all notes which aren’t “tagged” by a Playing Techniques. So I had to add “Nartural” in my expression map together with my new custom playing techniques.
The only thing which stays strange is, that the “Playing Techniques” lane in “Play”-Mode most of the time shows “Natural” even though the playing techniques in my score work correctly doing their defined program changes. I would love them to reflect the score…