[xps]

The most important thing is to figure out the options coding of each of the module that is required to support ACC. Camera, radar, instrument cluster, engine controller, idrive, steering controls and the brake controller.

This may be an impossible task if for example the ACC control unit expects the transmission to provide gear information as "D" (drive) to activate ACC which is not possible in a manual unless you create a proxy module to duplicate the automatic transmission messages.

1st step would be to understand what is needed to retrofit ACC on an automatic g87 without the ACC option from factory. That can give some definitive actions towards the goal.

Not to discourage you, but this seems too long shot to be worth the time.