We recently migrated from SVN to git. Unfortunately, only the trunk was migrated, but the branches for bugfixes on production revisions were not migrated. 8-{ Is it possible to migrate the missing branches now, months later? Unfortunately, doing the full migration again would be annoying, since we have been working on the repository for a while.
You could still make the full migration again, so you would have two repositories: One with the full Subversion history and all its branches (let’s call it “archive”), and one with the old trunk and development on top of that.
Then you could add the archive repository as a remote to the new one and manually integrate its extra branches into the new one.
It’s possible that some of the commits get duplicated that way but you could either live with that, or rebase those Subversion branches on top of the previously imported trunk commits.