How can I submit pull request to other's plugin at wordpress.org?
No, the wordpres.org SVN repositories are not for development and there are no plans to change this. So you either have to ask on support forum, or if the plugin is on an open GIT repository search for it on GitHub.
WordPress.org repository uses SVN, not Git, so there are no pull requests.
Only users with contributor status can access plugins to commit changes. Each plugin has one contributor by default, but more contributors can be allowed to each plugin.
Some plugins are developed on GitHub, and only released on WordPress.org when ready.