If have not found anything in the Data List, but I do not know how the Genos works.

What MIDI messages does it transmit when you change these volumes from the panel?

Yes, the Genos does not transmit anything during adjusting theese volumes, otherwise this would have been "easy"

I think I need some "undercover" midi commands to reach this function, it is not easy to get ?

Still no-one has been under the surface on this mashine ?

Jens,

You are probably asking this question in the wrong place.
This forum is about the MIDI standards and how they are supposed to be implemented, not about how an individual manufacturer builds their machines.
Nonetheless, many people here (who are not part of the standards institute) try to answer what they can.

Why don't you try a forum which specialises in Yamaha keyboards such as 'yamahamusicians.com/forum' or 'www.psrtutorial.com/forum'?
There you will find many experienced users of the Genos who might be able to help.
On both fora you will have to navigate a bit to find the correct section.
Post a query there, but don't expect instant answers, many are working musicians and have limited time.

JohnG.