Not that I know of, no. The market is pretty small. I can't imagine that Apple is selling many base stations at that price, and only a small fraction of those customers would be looking for a switch. I mean, a Mac Mini USB hub is one thing, but this market is orders of magnitude smaller.
I did a quick Google search, and the fifth hit was another thread on MacRumors, apparently with no more luck.
One option is getting another AirPort Extreme and turning off routing, DHCP, and wireless making it into a switch. Then you could place it on top of your current AirPort Extreme and connect them with a Cat 5e or Cat6 cable.