So - this works. Great. But the way is a bit long :-( Anyway, THANK you so much, pgroell!
Let me summarize the steps:
1) Open the SkyDemon outputed GPX File with a Text-Editor.
2) replace lines like <skd:level type="A" value="4500" /> with <altitude>4500</altitude>
3) remove <extensions> and </extensions> lines. This has to been done!
4) save the GPX File and close it
5) load up this GPX File here:
http://www.gpsvisualizer.com/map_input?form=googleearth right side of the window!
6) change these settings in the left half window, middle section: Altitude mode to "extruded" . choose also line width and desired colour and change the units (left window on top) to nautical!
7) klick on "Create KML File" on the right side. Wait.
8) open / import the produced Kml File to Google Earth
9) Enjoy ;-)