The process is not easy (and you need a "straight" Git repo: no merge, too complex branches and so), and the result is quite raw, but you're still able to convert the main part into SVN. Here are the steps:
- create an empty SVN repository: svn mkdir svn://path/to/repo/project/{trunk,branches,tags}
- clone it in a Git repo: git svn clone svn://path/to/repo/project --stdlayout . This will create a project directory, empty except for the .git information directory; master is the default Git branch created by git-svn that maps SVN trunk.
- cd project
- add the original Git repo as a remote: git remote add origin git://uri/to/git/repo/project.git
- fetch from it: git fetch origin
- create a local branch for the remote master: git checkout -b old_master origin/master . Note that master branch is already present, so we define another one.
- being on old_master we rebase this branch onto master: git rebase --onto master --root . Doing this, we have "moved" all our commits from old_master to master
- and push all the commits into the SVN repo: git svn dcommit
UPDATE: added 'cd project' step 2