Is it possible to create your own Fn+Key combinations on Windows 8.1?

Solution 1:

TL;DR: No.

Yes, but not natively. AutoHotKey provides a powerful way to do this and other input mappings. This thread provides info on how to map Fn.

Solution 2:

Generally, no. The Fn is handled by the keyboard microcontroller not the operating system.

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, some manufacturers, like Lenovo, perform this mapping in BIOS running on the main CPU, allowing remapping the Fn key by modifying the BIOS interrupt handler

https://en.wikipedia.org/wiki/Fn_key#Technical_details