Hi all, I found out how to do it! Now for people searching for the same answers I'll write up what to do.
1) Connect the MFC-10 Midi out to the Midi in of the PSR-2100
2) Turn off the MFC-10, hold Memory-edit and FC-edit down and turn on the MFC-10 again (for a reset back to factory settings)
3) Press down the Memory-edit and FC-edit twice. Now change the MIDI channel to C15 and use the Write/Exit and Yes buttons to save these changes.
4) Tab the first button you want to use, hit the Memory-edit button, change the status to F0, hit the Memory-edit button again and change the data to a c number at page 15 of the manual. Use the Write/Exit and Yes button to save the button.
That's all. I have now main A, B, C, D, Ending 3, main A, B, C, D with auto fill in included and a Break.
Now I have to find out how to store it to the memory of the PSR-2100 and I'm completely happy with this config.