In my experience, it is more common that flights get up-guaged than an extra section gets added. I've seen this happen quite a few times in stations like ORD and MKE when weather hits and flights end up cancelled (for example a few years ago on ORD-ATL, a 764 got subbed in for an M88). Again, this is generally only when one station or a small region is hit. When most of the east coast gets inundated though, the story may play out differently.