Fine to do for getting off the thing if it stopped while you're on it but no, they don't just become stairs when they're stopped. Go look up escalator brake failures if you want to see some horrifying videos of how that can go wrong.
Saw it a few years back and don't care to look for it since that shit scares the shit out of me (used to be a ski lift operator, rollbacks become a phobia in that line of work) but it was a large amount of people marching up the thing like it was a set of stairs when it just gave out.