Let's say I have a repository
git checkout -b userblah-test
git pull https://github.com/userblah/myproject.git
I think you're probably looking to perform a merge, which will take changes from one branch, and 'merge' (combine) them with changes on another.
Assuming you want to make changes to the proposed code:
git checkout -b userblah-test(checks out a new branch named userblah-test)
git pull https://github.com/userblah/myproject.git(pull in proposed changes from userblah)
git add .(the . adds all modified files)
git commit(commits the changes - it's important to note that you are committing your changes to the current branch only, i.e
git checkout master(checks out master branch before merging)
git merge userblah-test(merges all changes made on
userblah-testwith current branch)
However, if the pull request has been submitted through GitHub, I suggest you inform the original author about changes that you would like to make in the pull request comments. The author should be willing to make these changes themselves and add them to the pull request. You can then merge his PR in using GitHub's own web interface - see here for more info.
Pull requests are intended to promote discussion around code, if you're not 100% happy with the changes this person has proposed - tell them!