How to fork a non-GitHub repo on GitHub?

GitHub allows you to import an existing repository on Git, Subversion, Mercurial, and TFS.

See https://github.com/new/import

This is what I ended up doing:

1. Create new repo on GitHub

2. Clone the new repo

   git clone [email protected]:svnpenn/spoon-knife.git
   

3. Add source code from original repo

   cd Spoon-Knife
   git remote add upstream git://spoon.com/knife.git
   git fetch upstream
   git merge upstream/master
   

4. Push original source code to new repo

   git push origin master
   

5. At this point you can start committing your own source code!

source: help.github.com/fork-a-repo