Sonos has made no announcements about such a device. However, you can take your sonos speakers outside on temporary basis if need be. Alternatively, you use a connect:amp (or connect + separate amp) to power standard outdoor speakers.
I think part of the issue is that if you're going to place them outdoors in a permanent fashion, it's usually easier to lay speaker wire then it is to lay power lines to the appropriate speaker locations. If you would prefer to have something on battery and portable, the Bluetooth speaker market is already flooded. Then there is the problem that most people don't have good wifi outdoors.
Danny