To Add Custom Landing To a Stair

1. Select any stair with landing.
2. Select an edit handle at the midpoint of each segment of the landing.
3. Clicking this handle lets you insert a vertex at that location and move it to a new location changing the shape of the landing.
4. Tap the CTRL+Space key to make a curved segment(i.e. Arc Segment)
5. Pressing CTRL will toggle between a Straight Segment and Arc Segment

Custom Stair Landing for Straight Segment

Custom Stair Landing for Arc Segment