Custom polygon shape with Box2D/Chipmunk Collision

Hi

How could I create custom polygons and add them to the physics world in Box2D or Chipmunk?
For instance.

If I create a STAR polygon using CCPoint how can I add that so it is used by Box2D.

All the examples are of Sprites, you create a boundingBox of that Sprite and that is used by Box2D. If I understand it correctly.
How could this be done with Custom polygons?

Thanks