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.
Also, for those who are interested, here are the Stair Families that were created while doing this and the previous post on Nosing, Treads, and Stringers.