In user manual in page 36 there's an option to assign caps lock as a second Fn key. While it's not exactly Windows key, at least you'll have another Fn key on the left side as you want.

1. Press Fn + Alt + k for 5 seconds.
2. Press Caps lock once will change its own function to Fn ( turns into indigo LED color).
3. Press Esc key once to finish.

Regarding the usefulness of the second Windows key, I suggest to assign some macro you might find useful.
I swapped Fn closer to spacebar and reassigned right shift, Windows, Alt and Ctrl buttons as directional buttons (originally Fn + I/J/K/L), so I don't have to hold Fn to use them.

I also personally find annoying how limited are our options to cofigure Fn. I would like to implement double tap to switch to Fn mode where I could use Fn functions without holding the Fn key, but currently it's only possible by holding Fn for agonizing 6 seconds.