If i understand you correctly, what you need to do is extract the item you want to make into a light from the bsa, move it to the data files directory, in the same place it was in the bsa (i.e. if it was an exterior mesh, place it in the x folder, and be sure it has the same path as normal)
Then open up the lights tab in the CS, create a new item, and point to the mesh (you can't drag and drop because it wont be created due to using the same ID as the original static mesh)
You can then save and close, delete the mesh, and re-open the CS, and if the mesh was in the same place in the data files as it was in the BSA, it will still work as the CS beleives its pointing to the same file.
Hope that all makes sense