Sorry for my incorrect "FSUIPC terminology", I try to clarify :
I was able to "adjust" some buttons on the VRI Combo via FSUIPC using the tab "Key Presses".
On the left side there, one can select a button (e.g. WX on the Combo) and use that button for one of the listed commands.
When there is no suitable command listed, but there is a corresponding mouse click, then - on the right side of Key Presses - one can create the command via a mouse macro. On the PMDG panel, there is a mouse click spot to switch inches Hg to hPa. So I created a macro named "InchesHg/hPa" and now my WX Combo button works fine to switch from inches Hg to hPa.
I did not find a command Charly/STB (or similar) in the list, but the IvAp screen shows a mouse click spot (corresponding with the IvAp hotkey Ctrl+F5) for Charly ↔ STB, hence my question was if this could be used for a macro, let's say "CharlySTB". The next step would be to connect a Combo button (e.g. MODE) to this macro. Unfortunatly, pressing F5 directly or via the macro, makes FS hang.
I can now end as aboveor is there another solution via FSUIPC to work this out ?