git remote add other /path/to/XXX
git fetch other
git checkout -b ZZZ other/master
mkdir ZZZ
git mv stuff ZZZ/stuff # as necessary
git commit -m "Moved stuff to ZZZ"
git checkout master
git merge ZZZ # should add ZZZ/ to master
git commit
git remote rm other
git branch -d ZZZ # to get rid of the extra branch before pushing
git push # if you have a remote, that is
Reference: http://stackoverflow.com/questions/1683531/how-to-import-existing-git-repository-into-another