If by round muffler you mean the round exhaust tips on the 330i and by square you mean the exhaust tips on the m340i, yes you can change it easily.
You would need to remove the round exhaust tips and replace the inner and outer glossy back part on the lower middle of your bumper with the m340i parts, which is play & play. see below thread for pics.
https://g20.bimmerpost.com/forums/sh...07&postcount=3