These guys only show it with the sideing off and the flashing behind the upper siding.
I would just cut out enough to put the roof and you could put the flashing up first but if you are willing to accept a few leaks from time to time.
I suspect doing what you are suggesting will prove to be the hard way.
If the span is not to great and should be checked.
You would install something like a double 2x10 beam between the posts on the inside with heaver than normal hangers
Fill the gaps in the siding between the beam and the siding.
Bolt a ridge board thru the siding and spacers to the beam on the inside.
When installing the roof steel you will want to cut the top to fit the siding on the wall and caulk the joint.
I think it would be a whole lot easier to remove a foot of siding, then you would just use the beam as a ridge board.