Sounds like it's really a question for your local dealership.
However, the power drive unit is available as a internal replacement part (US$ 163 each in the US). It could probably be installed in each mirror but would be costly since it would require complete disassembly of each existing mirror and then completer reassembly. (lots of labor charges).
Alternately, you could order complete mirror assemblies with new housings, internal parts and glass. (probably over 250 euros each). Either method would require reprogramming of the main controller and iDrive to recognize the new LIN bus component. Complete new mirror assemblies come with primed housing covers only, which means they would have to be painted.
With the cost of painting, removal, replacement, and reprogramming you're probably into something like 1000 Euros total cost, if your dealer is willing to take it on.
Just a very rough estimate to prepare you for the dealer's response.
Adding "simple" options after delivery is always much more costly than [doing it] before the car is completed.
Remember, the standard mirrors do fold manually for when you're parked on a narrow street or in a small garage.