[Tricky2]

It should be possible, but costly as the parts to dealers are more expensive than at the factory, and there's the labour of removal and refitting as a one off, a lot more time than initially installing it.

On the mk2 Tiguan forum, a dealer forgot to order the electric tailgate for a customer, and elected to retrofit it themselves at their expense. They did a very Good job on it, so it is possible.