The petrol TSI was never sold in the UK with 4motion (as far as I am aware), and this is my biggest regret from buying the TSI, and has been bugging me ever since. I have even resolved that, should my van get written off (heaven forbid), then I would look for a 4motion TSI in Europe to replace it.
I am sure that it is possible to retrofit 4motion to a 2wd TSI, but just how viable would such a task be?
The mechanics are probably relatively simple: transfer box (could that attach to an existing DSG, or would a suitable replacement be needed?), differential & prop shafts. But the electonics might be the sticking point. Could the relevant modules be integated into an existing system?
Or are the differances so vast that such a job could only be done as a "money-no-object" project?