yea i have a itr muffler on my greddy pipe. i had to chop up the itr muffler to make it fit. i think, AM NOT SURE, its because the itr muffler is made for a coupe.

also, greddy uses a different flange than honda...i know honda uses a donut thing, and greddy doesnt.

if you want to just bolt on the stock muffler for that certain trip, not permanently, im sure you can without a lot of trouble.