I would recommend either a replacement or to weld hole shut. In my experience with catalytic converters; because they "re-burn" the gasses passing through them, when they are functioning properly, they develop very high heat approaching or exceeding the temps of gasses leaving the combustion chambers. Because of this the converters are much hotter than most other parts of the exhaust system and probably exceed the limits of muffler repair material. Also due to the large temp swings the converter shell will probably slough the bond with the repair material and repair will fail. Just my thoughts.