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.
Encourage you to look at the bigger picture. What happens when there’s an official extension by a 3rd party vendor SDK that defines a dependency on the proprietary language server/extension.
It’ll work fine for “Visual Studio Code” but all these OSS forks (inc recent venture capital funding for AI ones) gets wrecked.
It turns into a game of having to iterate through the graph and trying to convince official SDKs to change their product direction.
Such "3rd party extensions" don't get the "verified publisher" icon (or at least should not get it ;).