One way to do this is to make a copy of the original scene. Place your "powered-up" actor in the place where you want it to start the level. I'm assuming you know how to transition scenes, so: transition from the original scene to the secret level scene, and transition from the secret level scene back to the copy of the original scene.
Of course, if your actor has health and all that, it will be reset. If that's an issue for you, you could probably work around with the use of attributes.