My usual workflow when working with git, is something like this:
git help remote
should show you what you need to know. I think what you want is
git remote add [remote-name] [remote-url]
# Set a local branch to follow the remote
git config branch.[branch-name].remote [remote-name]
# Set it to automatically merge with a specific remote branch when you pull
git config branch.[branch-name].merge [remote-master]
You can also manually edit .git/config to set these up.