Hello Basti. I am using this setup since years, its great. So what you need is a Kemper, an iPad (even an old one will do the trick), a Yamaha MD BT 01 wireless midi Adapter, the forScore app and a second app called midi wrench (its free).
Connect the Yamaha to the midi in and out ports of the Kemper. The Yamaha is powered by the Kemper. Now you have to make the iPad see the Yamaha. You can not do this with the iPad and the forScore app only. Switch on bluetooth on the iPad and start the midi wrench app. In midi wrench go to settings, then to bluetooth midi and select the Yamaha. When there is a connection you will see all the mididata that is been sent in the right monitor window of the app. Let the app run in the background and start forScore.
In forScore you can add a specific command to each score. Add a program change to your score. Every Slot in every performans has a specific program change. Please read the forScore manual and the Kemper midi manual. Everything is explained there. The main trick is the midi wrench app to let the iPad connect to the Yamaha.
Good luck its worth trying.