There are aftermarket as well as BMW options for retrofitting. If you check the DIY area you should find threads. Aftermarket will be cheaper and probably easier than retrofitting a BMW system but I believe both can be done. With the right aftermarket, you can use the buttons on your steering wheel. Bavarian Soundwerks is another potential source.

I put in a high end (but now obsolete) Garmin GPS with bluetooth in my e88. I also put in their remote microphone and it works well. I have it mounted on a bracket BMW sells for their "Portable Pro" system.

Jim