I'm in the middle of a paddle shift conversion on 996 Tip. I've had to buy a manual car steering wheel and some AMG paddles. The wiring is simple as the paddles or thumb things are just switches. On the tippy you have up/down on both sides, when adding paddles you just eliminate one on each side.
I found several web articles on this subject. Google is your friend.
Here is the thread I'm working from:
911uk.com - Porsche Forum : View topic - Complete DIY for tiptronic to paddle shift conversion