You can use it the other way. It's a little more awkward, though. Go to the github page: https://github.com/schacon/hg-git and scroll to the very bottom. Read the section on the "git.intree" option. After setting that to True, you can pull from a remote hg repo into your local git repo by doing "hg pull" (or "hg clone") followed by "hg gexport". Then do some work in git. Afterward, you can push from your git repo to a remote hg repo by doing "hg gimport" and then "hg push". You may wish to write git command wrappers git-hgpull and git-hgpush to automate those tasks.
I should note that I haven't actually done this myself.
I should note that I haven't actually done this myself.