The general problem with doing conversions this direction is that Git repositories can contain more than just a linear history of revisions, as Subversion would expect. Multiple ancestries with divergent histories and frequent merge commits are all possible, which can’t be easily represented in a Subversion repository.
For simple cases where you do have a linear history in your Git repository, you can use git-svn dcommit to push the lot up to an otherwise empty Subversion repository.