Many many thanks for your answer. I have to read slowly across the whole explanation and configure my HOTAS. I prefer to do it from BMS instead of CH Control Manager as you said.
There is a callback HotasPinkyShift which can be partially set in game and partially needs to be set outside the game. It raises all the button numbers by 256 when held and does the same thing as SimPinkySwitch when tapped quickly. This provides a shift function when held.