I have not done it yet but was considering a lean-to on the side of my pole shed also. Just thinking how I plan to do mine may not be the best way, but I want to follow this thread.
On the eve side of my 14' wall 56' long shed I was going to put in poles like the original shed about 14' away from the wall. I planned on building the wall like the original shed wall.
Along the side of the existing shed I would run a 2X10 along the length of the building to hold the joist hangers for the 2x10 rafters that run from the existing shed to the new wall. I would run 2x10s every 4' but not sure of that. I have a lot of snow pile up on my existing shed.
Then nail 2x4s over the rafters the whole length of the building 2' on center and this is what I would nail or screw the roof panels on. For the roof panels, I planned to loosen up the end of the roof panels and slip a piece of roof panel under that a foot or so and extend it out to the new wall so the whole roof would be the same pitch. That's the plan, but that may change, depending on what I hear on your thread.
I just priced what it would cost and it would be about $4000 for just the materials.