the hooker system bolts like OE with the large 4 bolt flange and includes a 2 1/2 test pipe that is the same size as the stock downpipe , I assume your talking about the 3" poston stock type downpipe that uses the stock elbow at the turbo.
you could cut the test pipe in the kit by the flange get a 3" cat if you are subject to inspection and have it welded to the flange .
you could and have an 18" length 3" pipe with a swagged end attached to it and youll have a 3" test pipe.
you could buy a 3" test pipe or even one with a dump
you could get an adapter from 3" to 2 1/2" to and use the hooker test pipe or your current cat but it will be a restriction .
whatever you decide to do get a stainless band clamp for the connection of the cat/test to the down pipe and you wont have any trouble removing it and seperating the pipes at a later date