Meanwhile I found a solution that comes pretty close to the intended effect. First I didn't see it, because it's "hidden" exclusively in the stereo section of the Pitch/Synth effect list. As user who mainly goes by 4CM directly into an amp, I am used to the mono part only ...
All I had to do, is opening a second signal line with output XLR from which cables run directly into the mixer. In this line there are just 2 effects: Pitch/Synth - 3 OSC Synth and a compressor. I had to fool around a bit with the OSC settings but now I got a real fat sound by doubling the guitar strumming with a kinda syntheziser sound. Cool!
I would have marked this question as answered, but could not find the option, how to do this ...
Axxxeman's post in Need some expert help on sound topic was marked as the answer