My OP sucks, what I mean is I need to waterproof it from the inside. It's just that every bathroom I work on with a window in the surround results in a nasty moldy nightmare down below. I just want to seal it properly to the tile inside. Caulking joints need maintenance that does not seem to get done over the years.
Ya, no way around some framing/siding repair. I just meant I want to keep the same RO and buy a window so I can piece the siding back together and not completely reframe as I need to do for glass block. Often the siding I'm dealing with is from the 60's or 70's with no way to match it.
I installed kerdi in my bathroom and used pvc trim to flush out the window. it worked really well. I ran the membrane right up to the trim and overlaid the tile to it. Caulked it up and have had no problems what so ever. Love those schluter products. Wish I had known about their trim pieces I would have used them instead of caulking in the corners and at the tub. I would not doubt they would even have a piece to use at the window trim.
Check their trim section and see if that would help you out.
I've used vinyl windows and returned the tile, tile substrate and the waterproof membrane to the window, caulked the joint of the waterproof membrane to the window with 3M Marine 5200 Adhesive/Sealant (Kerdi fix if I'm using Kerdi).
On the sill I bevel the tile substrate and the tile toward the shower.
I also use windows with a a good weep system to the exterior and brass operating parts such as rollers. (Harvey)