2 Options:
1. Tab select a single curtain wall panel, Edit-in-place, Family Category, change to a window, Finish editing.
This panel will become a window and show in the window schedule, but it will no have the width and height information. You also need to manually type in the mark.
2. Create a new curtain wall panel family, or edit an existing curtain wall panel family, go to Family Category and change it to a window, rename an load into the project.
Now, for an existing curtain, you can still do the option 1 steps and change a single curtain wall panel into a window, with the width and height information of that panel, but you still need to manually type in the mark.
If you create a new curtain wall type and define the new window type panel as curtain panel, Revit will automatically mark all the panels in it.
What I normally do is to mark all the panels in 1 curtain wall as the same mark, add new manual project parameters
(width, height and sill height) for window category, then I manually type in the width height and sill height in my window schedule. I don't like this method to be honest, so I avoid the curtain wall if I can.
Does anyone have a better method?