There is actually an invisible platform. Just go to Doodads palette -> Bridges & Ramps -> Invisible platform.
To make stairs, place the platform where you want your stairs to be (hold down shift while placing them in map, so you can place them when they clip through each other).
Once you have set where stairs would be, select all platforms, which are supposed to make one stair, hold down left CTRL button and then press Page Up/Down buttons repeatedly to move the platform up/down. This way you make manual stairs.
Repeat the process for every stair and you're done.
Note: The height of a destructible will be set back to ground-level whenever you move that destructible.