Sorry Cassp, but it appears as if it is dedicated as strictly a modulation wheel and can be used for no other function. However, you can control or vary the volumes of R1 and R2 using the the number 6 and 7 buttons at the bottom of the display when you are on the main screen. It's fairly easy to do and I have done this with some songs when I needed more volume than the default volume of those instruments. Of course, you can also change them permanently by using the Style Creator program, then renaming the style file to a user file.
Hope this helps,
PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!