2002 Hyundai Accent Exhaust Manifold Weld?

I've been told two things 1 the manifold cannot be welded since it is cast iron, and 2 just weld up the crack on the manifold that leaks-on the Hyundai accent the manifold is connected to the catalytic converter and real expensive to replace. My question is can I weld the manifold or not-? (Crack is completely around and makes the car sound like a race car). Help!