Hi Pfeuh,
I guess that the shop guy simply connected the button accordion keyboard via MIDI to the Genos, internally.
But the original Genos keyboard, to my knowledge, does not output a MIDI signal, unfortunately.
In addition to the circuit boards MK76L, MKC and MKH (= contacts & keyboard matrix) the signal goes through the EMKS circuit board (key scanner) and then, via a connector board named JTC, directly to the so called "EBUS" on DM (main circuit) board.
You probably will need at least the Genos Service Manual to do further things with that FSX keyboard...
Best regards,
Chris