You could, but it'd probably sound really bad. A muffler also has little (but an important role) effect on overall flow, so if you were going for performance you'd want to replace the pipe as well. As for modifying the muffler I'd give you a big thumbs down on that, as it'll just sound and look cheap, as people will now think "haha, what a loser with a hole in his muffler". I had a 'low' rumble before I got my exhaust too, because I had a basketball size hole in my muffler....sOOOOOOO I say don't do it

if you REALLY want to though, you could open it, and weld a pipe straight through it?