Best to ask high-end audio shop...
I believe this would be possible depending on the ohm rating of the speakers you are going to use.
There may be some options of replacing the under seat speakers or even doing a larger sub in the trunk by using a high to low rca adaptor.
We plan on doing a 12" competition sub in the trunk once our 4 series arrives.
The best thing to do would be to visit a high-end stereo shop in your area to see what they recommend. They may be able to use prior 3 series options that they may have worked with in the past.
Good luck and keep us posted!