making units walk on stairs is a pretty easy process
1)Make the doodad/destructible walkable, you can do this in the object editor
2)go to bridges/ramps and place an invisible platform small or normal, i usually use small it usually makes it appear smoother
3)after placing your invisble platform, raise the doodad so that the bottom of the purple box is even with how high you want the unit to appear, don't do it too low or else it makes the unit sink, but don't do it too high so that the unit doesn't appear to be floating, just do that all the way up the stairs, you can use the same process with walls if you want to make them walkable.
i have a pretty good example i could show you, it's of a coastal fortress i'm working on, but it's currently on TheSandWraith's computer and i haven't asked him to send it to me yet.
Hope this helped you.