I've done exactly this ~4 years ago when I briefly worked at a place that used Subversion, after an acquisition. I wanted to be able to dick around in my own branches, with proper diffing and tracking and all, without updating the server, which appeared to be impossible (more or less). There was a git-to-svn I could use but considering how easy it was to screw up other people's state in Subversion, it made me nervous. So I just worked in my own, local git then copied the files to SVN when ready to commit something worth sharing.