Originally Posted by UltimateBMW
You can't simply just put a SMG onto a manual transmission car. They are completely separate transmissions. Thus, in order to change your car from a manual into a SMG you would have to completely swap out your transmission which is a very expensive process. Transmission work in general can be quite pricey.
Add to that the software issues of the engine control unit and transmission control unit (may be integrated with the ECU). When you have an automated manual of any type the engine and transmission get intertwined in a fairly complex way. What this means practically is new software and or hardware is required as well. Not impossible, just adds the price and complexity.