Try to experiment with basic stair options and change between "start with tread" vs. "start with riser". Then go to Flight Structure Settings and set the parameters for Flight-Start so the stair fits the slab and floor finish hight. It does work, but needs some tweaking. It helps if you have a section through stairs, so you can measure some needed dimensions.
Archicad 4.55 - 25 | HP Z840 | 2× E5-2643 v4 | 64 GB RAM | Nvidia Quadro M5000 @ Windows 10 Pro x64