Could probably be doable with database fuckery. Make a second community locally with an identical name, then in the backend swap some IDs around and make it think the posts were actually in the new one all along.

Granted, I haven't looked at the DB structure, so I don't know how plausible it is. If someone tries it, don't blame me if the instance breaks, do at your own risk.

But that creates a problem with potential duplicate name, but there is even bigger problem. By export I assume You mean taking ownership so adopting this community into the new server, who decides that? First come first served does not look good in my eyes as a good solution. Since that community is stored as a copy in other instances it should maybe just go into read only mode for eternity

something like this exists for Mastodon users

I believe Mastodon allows you to transfer your followers list, but not any content you generated, such as messages or likes. You can export both followers and content, but at the moment there's no way of importing content to new instances, only your followers.