We've just got a single sliding door GH 6x8ft.
It came with a base and is Aliminimum (spelling?) and polly panes.
We've dug a trench about a bricks depth on it's side, put a layer of bricks on top and pointed them. We've left a gap in one of the holes in the bricks nearest to the garage door so if we want to run electricity to it we can.
The much beloved is in charge of any construction and he's going to run batons along the "wall" to drill into and fix the greenhouse. There's a small lip on the bottom of the GH frame and the wood should make it easier to square up the frame on the foundation.
We're going to totally loose the base that came with it.
I don't know if this helps you or not but I don't understand why you would want to extend the door?
If you're worried about a big drop either side of your foundation surely you can back fill inside the surround to raise the level inside, and have a couple of flags piled up outside as steps?
I might be wrong but I think you're possible making it tricker than it needs to be?