Program Keys without Existing FN Layer?

[LoveYouFyi]

I have a non-programmable keyboard that has dip switches.  I need to further control which key does what when using FN + "Any Key".  I need to assign PAGEDOWN to the key with the opening brackets: [{
The problem is, that key does not have an existing FN related key mapping.  I am using Karabiner for key remapping, but it seems that for using the FN modifier, I am limited to only re-assigning new keys to use with FN if that key came stock with an existing FN layer key mapping.  In other words, I am not able to re-assign the FN + KEY combo to make use of any key combination of FN + KEY unless that key came stock with a FN-level assignment. 
Is there a software you can recommend that allows me to achieve this?   

[Tactile]

On most keyboards of the type you describe the FN key is handled within the keyboard by the controller, which does not send the keypress to the computer. The computer has no way to know when the FN key is pressed.

One of these might help you do what you want.

[ddot]

Using the space bar as an FN key may be an alternative workaround.

https://geekhack.org/index.php?topic=51069.0

[LoveYouFyi]

Thanks for your replies, they help a lot.  Especially knowing the FN key does not register outside of the keyboard.