If you can use the keyboard's Fn key plug-and-play without installing a special driver (and in most cases even with such a driver), then it's a hardware Fn key which sends remapped key codes to the USB and you have no chance whatsoever of remapping it into an Extend key. The only flexibility left for you is remapping the key codes sent (such as arrow keys) and that'll remap all such key codes – there is absolutely no way of differentiating between a physical arrow key and an Fn-key generated arrow key for instance. They're the same key code, and that's all the computer sees.
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