Without having the body ECU programming in my hands, I couldn't say if it is technically possible. The body ECU sends the beep signal to the buzzer, as well as locking/unlocking the doors when it receieves the requests from the wireless receiver.
If the buzzer is already there for all packages, as I suspect is the case, then the body ECU is withholding the beep signal from the buzzer if SKS is not present. However, if the buzzer is simply missing, it could be added, assuming the pin on the body ECU that it connects to is trying to send the signal to a non-existent buzzer.
I would have suggested connecting the buzzer to the parking lights so it would go off when they blink, but then the buzzer would sound continuously if you had the headlights on.