I finally understood that the button "open image in external editor" was not only for selecting the lightroom executable (by means of the little
downwards arrow on the right), but also had to be clicked to actually invoke the import to lightroom, rather than using the "save image as"-button.
And then I had to set up the destination for file import in lightroom properly. I used "organize by original folder", this results in a new subfolder named
snshdrh2-P1XkOA to be created inside the folder selected below. The import wasn't quite automatic, I still had to select the picture. Also, I was not able to put the new images directy into the subfolder where the original raw images reside (and hence to put them into the same stack), but this subfolder is also fine with me.
Keep up the good work,