As far as I know the IXEG does not expose all the buttons and switches as commands by default, so that is why you won't find them when mapping your keyboard or joystick buttons. One solution would be to create a FlyWithLua script to create all the required commands for the overhead panel switches and buttons and then you'll be able to map them to a keyboard or joystick. But... this is not a trivial task unless you know FlyWithLua scripting and how to create custom commands.
So yes, it is possible but not so straightforward.