Found this on another forum, "I have had LiPo stored in -20C (-4F) without any problem."
I certainly wouldn't recommend this but yours should be fine. Just be sure they're back to room temperature before you charge them. I saw several posts that said not to charge LiPos below 50°F.
Other links I followed imply there could be a problem if they freeze solid but they didn't say at what temp that would happen.