You can't keep the BMW CD Changer, but you can use an Alpine CD Changer with the Intravee, the CHA-S634 is MP3 capable and is physically the same size as the BMW changer so you can use the same brackets. If you want more than one CD Changer type device (CHA-S634, KCA-420i iPod interface etc) then you also need an AI-Net switch.

Have a look at the FAQ on the Intravee forum, most things are covered :

Forum FAQ :
FAQ

Existing CD Changer:
Can I use my original BMW or Rover CD Changer with the Intravee?

Connection:
How does the Intravee connect up to a BMW, Mini or Range Rover?