Not sure if it actually possible and I know I don't have the skills to do this, but I had an idea of how to do modular stairs .
It involves a counter and then you change the counter to get rid of everything below the countertop and then change how the countertop reads so that it now reads as a floor tile that you can place any height. One flight of stairs could go from the above floor to the countertop/floor tile that is above the floor a ways and then another flight of stairs could go from the countertop/floor tile to the actual floor. No idea if that is possible and I don't have that much modding experience.
I dont even care if the animation is correct. I have deco stairs in my house with a no footprint chair upstairs. So, when I want then to go up, I just make them sit in the chair. This is how desperate I am to have different stairs.