well, since WheelForward and WheelBackward are simply ment as key_names, like f.e. 'a' or 'space', I think something like:
Code:
<Hotkey WheelForward>
 	<Sendlabel w1,w2,w3,w4,w5>
       	<Key WheelForward>


 <Hotkey WheelBackward>
  	<Sendlabel w1,w2,w3,w4,w5>
        	<Key WheelBackward>
should work. (of course with changing w1-w5 to the labels you use...)