The "web mvc scaffold" command that creates the controller and views for a single entity has an optional "path" argument that lets you specify the base URL for that entity (defaults to e.g. "person" for the Person entity). So you could set this to "admin/foo" when scaffolding the UI for the Foo entity.
The "web mvc all" command that creates the controller and views for the whole application at once has no such option, as it's intended for a simpler use case.
If you have existing controllers and views and want to rebase your application's URLs, I'm afraid you'll have to update them all manually.
Andrew Swan
"Now is the EJB of our discontent made glorious Spring"