Overriding admin login template


I’m looking for a way to override the Pimcore admin login template from a custom bundle. Specifically, I’d like to add a link to a single sign-on page below the login form.

I’ve been looking around for a solution for a while, but it seems there isn’t that much information to be found on this topic. I found some references in the Symfony documentation, but either I can’t get it to work or it doesn’t apply to this specific situation.
Another solution I found seemed to involve making the bundle an extension of the admin bundle, but I’m not sure that’s such a good idea, since only one bundle is allowed to do that and this is such a minor modification.