How to take someone’s PR and make your own based off it while keeping their commit:
git checkout -b pr/xyz
git pull git://github.com/rafase282/wiki.git update/pr-guide
This basically means pull from the repository Rafase282/wiki, branch update/pr/guide into your current branch pr/xyz.
Note: You might need to force push
git push -f origin pr/xyz
That should make a new pull request with the original changes, plus your own into a new pull request that will automatically reference and close the original one when merged.