If you really want to do that, don't do it by ruining your engine. Just buy one of those "kits" off the internet that installs a spark plug near the exhaust tip and some sort of squirt thing to make the flames.
ALSO: Catalytic converters burn off gasses the motor doesn't burn therefore there is nothing to burn at the end of the tailpipes. You can remove the catalytic converter or hollow it out if you have one, but there are laws in some states that require you to have a properly working catalytic converter on your vehicle. There are fines for not meeting these laws so due your homework before you tamper with your catalytic converter.