Using the Create Sketch tool within Stair by Component allows you to create custom stairs.
However, if you are creating any L-shape stairs with a Cast-In-Place Stair, you might encounter a problem that looks like your stair has been chopped off on the corner.
This is an issue within the Monolithic Run system family. More specifically it seems to be a related to having the Underside Surface set to Smooth.
When changed to Stepped, the stair once again looks correct in 3D views.