Adding to someone else’s pull request on a GitHub project that I do not own

Question

You can check out the branch and re-submit a modified PR (giving credit to the original, preferably).

You can also issue a PR to the PR author:

git remote add userb https://github.com/userb/name.git
git fetch userb
git checkout featurebranch
[change and commit]
git push userc featurebranch

When you create a PR, GitHub lets you choose the base branch – so you can choose the fork and – if you want to – request changes to the PR.