Hi kefegan,
First of all, the BT3900U does have a socket on the back for use with stalk controllers, so that's a good start!
Whether you can do this or not depends on what car you have. If you have a make of car for which adapters are available then basically you you will need two interface leads: one that is specific to your make of car, and a second for your make of stereo (Sony here of course).
Sony don't make these leads as far as I know so you have to go to a third party adapter manufacturer/supplier.
I've used these guys before (http://www.connects2.co.uk/) and found them to be very good. Have a look at the website and you should be able to find the two bits you need (the steering wheel control adapter - for your car, and the patch lead - for your stereo), but I'd recommend ringing up as well to make sure of compatibility as car manufacturers have a habit of changing wiring configurations on their cars and not telling anyone!
Hope this helps.
