Bearding in late September during a warm spell — normal, or should I add a box before winter?

Ontario, second year, three hives. We've had a freak warm week — 26°C today, humid — and tonight two of my hives have a beard on the front like it's July. A few hundred bees hanging off the landing board and up the face of the box, fanning. My first-year brain says "they're crowded, give them a super." My second-year brain says something about not adding space in fall, but I can't remember why exactly. Both colonies are on two deeps, heavy with syrup and honey, and I pulled the last super three weeks ago. Reduced entrances went on last week for the wasps, which I'm guessing isn't helping the airflow. So: is this just heat management, and is there any situation where you'd add a box this late? And should I open the entrance back up for the warm nights?