Generally, the Fn key is not a real key, not a modifier like Option or Shift, but instead is a hardware toggle, that toggles the meaning of the function keys between their function key behaviour and their hardware key behaviour, and in the case of the Touch Bar, presumably also toggles the behaviour of the Touch Bar.
But you cannot do things like “Fn-A”.
You can trigger on a modifier key (like Shift) using the USB Device Key trigger, but that does not really help you here, since you cannot restrict it to the Fn key (I’m not even sure if the USB Device Key trigger can detect the Fn key).