And here would be a short noob-guide for anyone coming here!
1. Go to "Audio". Click on mixer and create a new Audiobus (f.e. "Testaudiobus")
2. Click on the new bus and create the effect "Reverb"
3. Still in "Audio" create a value (f.e. "Reverbstrength") and set as minimum 0 and as maximum 100. As beginning value set 0
4. Go back to the new bus and click on "new automation"
5. As source take the newly created value (f.e. "Reverbstrength"), as field "send 1" and as type 100:1
6. Click on the button "new send"
7. Take as source the effect "Reverb", all other settings can be kept as they are.
8. Go to "General game settings"
9. Scroll down and set the soundbus to your new Audiobus (in our example Testaudiobus)
10. Got to "Scenes"
11. At the beginning of the scene where you want a reverb set an action to change the value "Reverbstrength" to whatever you want
12. Play and enjoy!
Hope not to have forgotten anything
Thanks again for all the help!