This is a good question. I don't think it is currently possible. However, I do think it's probably a good idea for the developers to think now (if they haven't already) about what happens to communities hosted on an instance if that instance is shutdown.

All of the posts and comments from that community from before the shutdown may already exist locally, but would local users even be able to keep using it? If not, might there be a way for the local instance to "adopt" a defunct community?