Paddle Shift Retrofit

on my 2004 645ci, I like to replace my existing steering wheel with one that has the paddle shifters, what has to be done to make the shifters work or can it be done at all?
I have found some info about retrofitting a 5 series with paddle shifters. I also called the dealer they told me that they had this request before and the dealer contacted BMW to find out of it can be done but the dealer was told it would not work, well, it may work but it is just too much to change and the dealer and BMW do not recommend this.
Finding a used steering wheel with paddle shifters is not a problem but what next?