As far as I know, you can buy the .max files for the Complete Character HD set separately, but that still is not an easy fix. In vizard, there is not much you can do to change the animations. One cumbersome way of reversing existing animations is to run the normal animation and save the rotation of every bone & the position of the pelvis bone of your Complete Character in a list. Reverse the list, and apply the rotation & position data to the corresponding bone of your Complete Character. See the code in the thread
http://forum.worldviz.com/showthread.php?t=4948 for some info on that.
On a side note: I cannot easily imagine that reversing a running forward animation will create a realistic looking running backwards animation. Could you maybe elaborate what you're trying to achieve in general? Perhaps there are other (simpler) ways.