You can add any repository already on your hard drive by dragging the folder in from Explorer. If that folder is a Git repository, GitHub for Windows will import all of the history and connect it to your GitHub Enterprise account.
Another great way to add repositories is to clone them directly from GitHub Enterprise. You can do that right in the application by browsing through your personal account or any organizations you belong to. You can also clone with one click from any repository page on GitHub Enterprise. Just look for the Clone in Desktop button.