From the Chromium devs: "we're putting the power back in the user's hands by allowing them to control where extensions are installed from. By default, the Chrome Webstore is the only source, but users and administrators will be able to add other safe sources as they see fit."
I don't know if the ability to add other sources has been implemented yet though.
>Enterprise Administrators: You can specify URLs that are allowed to install extensions, apps, and themes directly through the ExtensionInstallSources policy[1].
I don't know if the ability to add other sources has been implemented yet though.