upgrading the exhaust system and keeping the stock catalytic converter?

i was wondering if this was possible?
the reason for this is beacause people tell me that the aftermarket ones wont pass smog.
and if you guys know of any brands that make aftermarket catalytic converters which will pass smog, please tell me

if you live in california most likely a highflow wont pass there, but everywhere else it should be able to, but that just depends on the highflow cat. But yes it can bolt up to the stock cat, but you will need a flare made to reduce the end of the header to the proper size. But thats assuming the exhaust manifold that you bought is larger then stock