My understanding from your first post, is that you are not able to place light fixtures on a winder stair that is linked into the parent project you are working on. Because of this, you are looking for a way to bring the stairs into a family environment so you can place lights on it. What I am suggesting, is that you can place your lights/fixtures directly on the stairs - or any surface - if you make them Adaptive Families. That is, make each light/fixture family Work Plane-Based and Shared, Nest it into an Adaptive Family Template, and host the light/fixture to an Adaptive Point Work Plane. Use this Adaptive family to place instances of the light/fixture on the surfaces of the geometries in the project.
Open the attached “Linked Winder Stairs with Lights Behind Translucent Risers.rvt” (the “Winder Stairs.rvt” is Linked into this Project) and see how a light family named “Rope Light Single Light Source” is placed directly on the underside of each stair tread via an Adaptive Point Family named “Rope Light Single Light Source AP”. Both these families are accessible through the Project Browser, along with another 3-point Adaptive Family named “Rope Light” which parametrically arrays multiple instances of the light per placement instance. No instances of this family are placed in the project. Place some instances of it yourself to see how it works. Number of Lights is controlled via an Instance Parameter of the same name.
Lastly, because it is Shared, the Photometrics of the light source family can be accessed by right-clicking on the “Rope Light Single Light Source” family in the Project Browser and pressing “Type Properties…” in the drop down menu.