Can I submit commits to someone else's pull request in my repository?

On GitHub, the person who submitted the pull request has the option to allow updates from maintainers. If that is enabled (by default), you will be able to commit to the forked repository (and this change the contents of the pull request).

I would not consider this a violation of etiquette (since they allow you to commit changes), and it's more useful since you can merge the correct code into your repostory/branch.

The official documentation for this feature can be found at https://help.github.com/articles/allowing-changes-to-a-pull-request-branch-created-from-a-fork/ and https://help.github.com/articles/committing-changes-to-a-pull-request-branch-created-from-a-fork/.