The Spiral Stair tool has been implemented lately. Here is an introduce of the tool.
This tool enables you to make a curved shaped stair with ease. The way of creating a winding stair is very similar to using the Cylinder tool. Drag the mouse to create a basic form of the stair. Release LMB and move the mouse to raise the height of the stair. Press LMB to adjust the parameters in the inspector and then press SPACE to finish the whole process.
The left animated image showcases how to make a winding stair with this tool. Like other stair tools in UModeler the Spiral stair tool is also based on the rise not the number of steps so you don't need to reason the height of a step while creating. Actually it doesn't matter how many steps are in a stair. Instead what matters is to maintain the constant riser size of all stairs in a scene because characters could go up the stairs or not depending the riser size.
This spiral tool is going to be included in 2.2.2 version that will be released soon.