The answer seems to be: You can't.

Once a pull request is merged and closed, it is locked forever and cannot be reopened. If your pull request is merged, closed, then your changes are pulled out (via force pushing backwards to before the merge), you will need to add commits to the branch and create a new pull request, copying all the details over and probably providing a link to the original pull request to manually save the history.

Might be a nice feature request for future GitHub.