I know there must be a module that you can buy, I've been searching some sites, lazily, and haven't come up with anything just yet.
I know that you can always bypass any factory system by running new speaker wires to new speakers, and this being an eleven year old car, that might not be a bad idea. I never worked on many BMWs when I was installing in MS though, it was kind of a poor area of the state and we didn't see many new or used luxury cars and there were no dealerships near us.
I'll tell you though, some of the Bose integration units and others that I've used haven't always worked all that well. Even in my brother's Avalon, it never really integrated with the system all that well and would go static every now and then. I almost always preferred replacing everything. I can't tell you how I feared seeing the dreaded Bose emblem in all of those stupid cars, a big hassle. Just thank yourself that you didn't buy a Cadillac Avante.