Unfortunately the "special key" (in other words, the "modifier key") on the SteelSeries 6Gv2 will always be a "modifier key".
This is because SteelSeries has hard-coded this button. It'll not be possible to re-assign it.
Here's a quote I found in the SteelSeries FAQ, if you're interested:
Some of my multimedia keys don't work and I can't re-assign them for any other function.
The 7 Multimedia control keys (Volume Up/Down, Mute, Back Track, Stop, Play/Pause and Next Track) are designed to provide standard Windows® multimedia functions. They currently support Windows® Media Player version 6.4 or higher. The Multimedia keys are hard-coded to perform their respective functions, and cannot be reassigned. If you're using Winamp you must enable 'Global Hotkey Support' in the Winamp properties.
If you dig deep into the Internet you may find a tutorial how to do this physically, but you'll have to solder, etc.