将现有的git repo推送到不同repo的新分支

I'm using git-tfs to migrate repositories, and due to a quirk of the tool I have to clone/convert a branch to its own repo. How can I push this separate repo, which has its own history and everything, to a branch of an existing repo? I can start from scratch and try to push everything at once if that's easier.I'm using git-tfs to migrate repositories, and