I don't think this is possible. What you can do, however, is use an intermediate Autohotkey script to accomplish the same. You would then set your SI gesture to execute any hotkey you like (such as control-shift-u and as control-shift-d), and create two hotkeys in an Autohotkey script that convert this into volume up and down:
^+u::Volume_Up
^+d::Volume_Down
In Autohotkey, ^ is control, + is shift, % is alt, and # is the Windows key. You need to copy and paste the code above into an empty text file, save it as a .ahk file (like "Volumescript.ahk"). Then install Autohotkey from their website, double-click your Volumescript.ahk file, and the hotkeys are active.