The Zero-G is fine, although very little gravity would work better for walking.
What you can possibly do is let the user use his normal jets to go up and use that jetpack addon to go down.
Changing it to go down is probably nothing more than changing a variable from 180 to 360.
Well, once you slow down after jumping, you are able to walk.
I also have a mod in which you press the crouch button you start going down faster.
