I looked into the possibility before. I see three major problems.
1. The flanges on the bottom of the cats would not match up with the flanges on the downpipe section of the 6th gen header.
solution: cut the flanges off of the OE j-pipe and have them welded onto the 6th gen downpipe
2. The tube length on the 6th gen downpipe section (from what I can tell) may be slightly longer and cause the header to hang lower than it should.
solution: cut the tube back slightly to correct
3. To buy this section it costs $400 from comptech, and ~$200 from OBX or comparable (I already asked them ) and given that there is no guarantee that this would even work, and the fact that custom fabrication would be required anyways, its probably cheaper to have one fabricated at a shop.