I've done that.
Just choose "sloping wall" and give the ending point (or starting point) a lower number.
For example: Start: 125, End: 55 (or the reverse, depending upon how you've placed your stairs). The numbers I used were for the "Grand Staircase" model.
See this screenshot example: http://i64.photobucket.com/albums/h186/gymnart/Stairs-slopedwalls.jpg