Hi folks,
Thanks to Tobi and tomatpasser (the author of the fabulous GPIO plugin) there is a solution now.
Tobi worked it out and i’m allowed to translate the instructions to the forum

Using Saiyato’s rotary encoder plugin you not longer need at least the next, previous, vol+ and vol- keys
1.) Create playlists for each radio station (or album) and name them “key1” “key2” “key3”…
2.) Edit the file “/data/plugins/system_controller/gpio-buttons/index.js”
sudo nano /data/plugins/system_controller/gpio-buttons/index.js
For example to change the “next” and “previous” buttons replace the code beginning at line 254 this way:
[code]//next on playlist
GPIOButtons.prototype.next = function() {
//this.logger.info(‘GPIO-Buttons: next-button pressed’);
socket.emit(‘playPlaylist’, {‘name’:‘key1’})
};
//previous on playlist
GPIOButtons.prototype.previous = function() {
//this.logger.info(‘GPIO-Buttons: previous-button pressed’);
socket.emit(‘playPlaylist’, {‘name’:‘key2’})
};[/code]
3.) Go to the plugin settings and define the GPIO pins you use.
Don’t miss to have a look at tomatpasser’s graphic on github to determine if you have to connect your buttons to 3,3V+ or to ground.
https://github.com/tomatpasser/gpio-buttons
In my case i used GPIO23 and GPIO24 (Pin 16 and Pin18) - that need to be fed by 3,3V to “pull up”.
4.) Restart your system and you’re done 
Interestingly switching between radio stations works noticable faster than doing this on the web interface 