Here's an article you might find interesting.
I think the place to start would be to frame the windows out in wood. Then go from there.
Two ways. Kelli's suggestion is one. Frame the windows with wood and build the greenhouse around them. Or build the greenhouse out of wood with openings for the windows.
I would frame the structure and then mount to the frame.
Just curious where the one in the picture came from?
this family got old windows and made a greenhouse
You will need to establish a waterproof, level, and durable supportive base for your greenhouse, so that the building itself is off the ground.
Without this being in place first, any structure that you try to create will soon sink, shift, rot, and disintegrate.