I'd forget about converting to the 18 button control.
At the end of the day it's just not worth the money, hassle, headache, pain in the ass, and aggrevation. You'll always have something go wrong or not working.
Either live with the 11 button control, which isn't bad at all ... or sell the car and buy another with the 18 button controls.
Just my 2 cents
1996 E36 M44 318i 5sp LSD