Fn is not a normal keyboard key, such as the modifiers Ctrl, Shift and AltGr. For a standard modifier key, the microcontroller inside the keyboard sends a scancode for the modifier itself, which is then interpreted by the operating system and combined with other simultaneous key-presses. The Fn key is a form of meta-modifier key, in that it causes the operating system to see altered scancodes when other keys on the keyboard are pressed. This allows the keyboard to directly emulate a full-sized keyboard, so the operating system can use standard keymaps designed for a full-sized keyboard. However, because the operating system has no notion of the Fn key, the key can not normally be remapped in software, unlike all other standard keyboard keys.
While it is most common for the Fn key processing to happen directly in the keyboard micro-controller, offering no knowledge to the main computer of whether the Fn key was pressed, at least one manufacturer, Lenovo, performs this mapping in BIOS running on the main CPU. This allows remapping the Fn key by modifying the BIOS interrupt handler.
Answer to the question: Remapping not possible via operating system but only via BIOS (in some cases).