I mean, seriously? They should be able to, they should be able to even climb downspouts or what are they if they are not too demoralised.
You tried using a stepladder to get an NPC out of a sinkhole too?
If that happened I’d just reload the last savegame. I want them to climb the roof of my base (garage) so I can order them to build some smoking racks / windmills on there.
I think I’ll just order them to build normal stairs, but it’ll take a NPC 12h to do so.