The only 'extra' button I use as a CSS button is the thumb one. (quick switch weapons).
For that I just assigned it in the normal way via CSS options selecting the action etc.
Or...
Here's where I start getting outta my depth.....
There's the 'Select Program' bit in the 'Set Point' software.
From what I'm guessing having never used it personally....
is you can set the mouse to have different configs to diff progs.
So you could set up a CSS profile, and set the (extra) buttons you want to use to keystrokes.
Then should just be able to set them in CSS in the normal way.
Hope that helps.