For this, you can do it manually
Add a fork as your new remote repository using:
git remote add otherfork git://github.com/request-author/project.git
Then you need to fetch the remote
git fetch otherfork
For pull, you have 2 options:
One, you can rebase the branch on which your pull request is formed. But this will ignore the eventual commits between origin and pull request.
git rebase master otherfork/pull-request-branch
Or if you need all the commits in the pull request, then find SHA -1
And use cherry-pick
git cherry-pick <first-SHA> <second SHA>
Thus, you can unmerge upstream pull requests from other forks to your forks.