Dumping as in putting turndowns instead of running it out the side of back? Also are you replacing with a new muffler or what?
Really any exhaust modification won't produce any real gain or loss in any area except sound. So whatever you do you should be safe.

 

Seiously why? all you will gain is obnoxious noise POed neibors and irate police oh and a head ache as well as hearing damage

 

Did you read my first post? Yes you can do it but it won't sound the greatest. The resonance usually makes for drone. MPG's won't be affected unless you use a too large diameter pipe or cut it too short. Any specific questions about it?

 

Yes do u have to replace the catalytic converter and how loud is it

 

No you do not have to touch the cats to do any system unless you want to. Most generally a dump system exits before or behind the rear axle with the outlet hidden, with the modifications beginning at the stock Y. Sound volume depends if you plan to run a muffler with the system. "Dump" is simply the type of exit, not the whole system.