Everybody can do that themselves, if the original maintainers don't want to (which is a security problem ;):
If you are not the author, we suggest you first reach out to the author with an issue in their GitHub repo to request that they publish their extension to open-vsx.org. We've drafted a template with suggested content for the issue.
Such "3rd party extensions" don't get the "verified publisher" icon (or at least should not get it ;).