Can I use the greddy or any catback piping with the stock exaust or will I have to buy a new muffler?

 

When you replace the piping, you are probably replacing the stock b-pipe (intermediate pipe). This pipe connects the muffler to the catalytic converter. Cat-back systems like the Greddy and A'pexi contain both the muffler and b-pipe. If you want to keep your stock muffler, use just the b-pipe. Stillen and Courtesy Nissan sell them.