The Berk exhaust that I installed is a full system that goes all the way to the front of the car. The Berk pipes were a little bigger than the stock pipes and were welded when they were installed.
Since your just changing your the muffler, you need to be confident that pipe follows the same angles. My advice is to go to a muffler shop. They will have tubing expanders to create to create slip fit install, or they can make adjustments to the pipes if thing don't line up perfectly. Call around locally for a shop that does custom installations (non-chain shops are the best bet). You could also look for a shop that does Hot Rod builds or restoration work. It shouldn't cost too much, especially compared to what BMW parts cost. A whole cat-back for my Camaro costs less that just a muffler for the 1.
Bill [92 Camaro RS (Autocross ESP / Trackrat) | 09 BMW 128i Sport Package]