It's not readily available and I can't think of a way to emulate or work around it using Sprite Kit's built-in physics.
That said it's fairly easy to just ignore Sprite Kit's physics and instead embed Box2D (or Chipmunk) in your app. You can step the world in the scene's didSimulatePhysics message (gets called even with physicsWorld property being nil). You can store references to nodes inside the body's userData, and after stepping enumerate the bodies and update their node's (obtained from body's userData) position and zRotation properties.
Those are the essential steps. Just because Sprite Kit has physics built-in doesn't mean you can't use other physics engines.