What kind of flange does the cat-back have, if any? Or is it just a raw cut?
Cheapest option would be to use a sleeve clamp and cut off the flange(s), but definitely not the best.
Going all out, you would need to either add or replace whatever is on the cat-back side to a flange that matches the DP. Should not cost more than $50 to buy the flange and have someone weld it on for you, provided you bring the piece off the car alright marked for placement.