Hi!
2 options in my opinion:
1) (I use this one): Setup a modifier for the paddle lever on your WH (modifier is like SHIFT/ALT/CONTROL) and set the ZOOM IN SLOW/ZOOM OUT SLOW to the PADDLE+DMS UP/DOWN (or any other, whichever you want). If you need me to clarify this more let me know :)
2): In your options, you have absolute forward/backward movement (see axis commands in the options) set to a TrackIR axis (can't remember which one). Set that axis to zoom view. That way you move your head forward, you zoom in.
Hope this helps,