To some degree you can. Though I believe, and I'm not 100% on this(but only because I've never had a Fed. hub), that the non-drive top hat in the Fed hub has an ID of 14mm and only fits Geisha hub shells(NOT GEISHA LIGHT) and Fed hubs. The Top hat from the Geisha Light has an ID of 15mm and will only fit in the Geisha Light hub(the OD is smaller for the Geisha light) So you can , but I would think you would need to spin the OD down a little on the Fed. top hat washer so it can fit in the Geisha light and allow the non-drive bearing to seat in it's seat.
I will say, many on here seem to make the switch, but I'm skeptical due to the numbers and my experience with the hub and trying the Fed axle myself on a Geisha light.
One more thing... I'm also not really sure how much you really would save, don't both have Aluminum shells? It's the axle that save the weight in the Geisha Light, or at least that's what I've always thought.