The best you can do is:
When they are created give them a wind walk ability with:
-10000000 transition time
-10000000 duration
-a custom buff which has no icon and no text, so is invisible
Force them to use this ability and remove the ability. Voila!
drawbacks of this method:
-units will never crit
-units cant walk through items put on the ground
-Dont give those units another wind walk ability, it will override when used
e: probably biggest downturn is that when the buff is removed the effect ends too, so you have to redo the process..