This would need to be done before installation as you do not generally have the clearance to use the fence. I think it could also be a problem if you needed to replace a window as you would have to cut out the sill as it generally extends past both legs of the trim. I have used the Domino to make trim units ( head, legs, and sill) sometimes these also include extensions. I find it easier to attach them to the window with pocket screws.