Fn is an immortal topic 
... although I don't know exactly what the topic is in this case.
If anyone wants to use fn as a modifier with KM (without BTT), then in macOS it is sufficient to switch fn to control. This makes fn the physical right control key, which most keyboards do not have.
This allows for two shortcuts with the same letter. For example:
Left control (normal control key) + A = select all
Right control (previously fn) + A = something else.
All you need is noisneil's macro