Instead of telling git-svn about a remote branch created, you can create a remote branch manually. For that you could use the following commands:
git config --add svn-remote.newbranch.url https://svn/path_to_newbranch/
git config --add svn-remote.newbranch.fetch :refs/remotes/newbranch
git svn fetch newbranch [-r<rev>]
git checkout -b local-newbranch -t newbranch
git svn rebase newbranch
The above commands will create a remote branch after you fetched the repo.