I have a very old project that includes the source from another project directly, instead of linking it as a library. Back in the bad days, when I was keeping everything in CVS,
Since the hg-git does mention that submodules are not supported yet, that leaves only a manual option:
hg gpush
from within your project. For example:
$ cd hg-git # (an Hg repository)
$ hg gremote add origin git@github.com/schacon/hg-git.git
$ hg gpush
That Git repo will represent your submodule, but if you modify and push that submodule, you will still have to pull from that Git repo to the actual Hg repo.
Other great git-hg commands are listed in this "rosetta stone".