This can be done with a conditional statement and some spline IDs, as follows:
- Set the stair spline segment's material ID to 2
- Create a new Conditional operator and wire it to the Evenly input. Turn on Material ID and set it to Equal 2
- Wire an Empty segment to the True input
- Wire the existing compose operator to the False input
- Select the generator and set the Properties > Rules > Default > Mode to Adaptive
I hope that helps,
let me know if you have further questions.