seing as you have the dynamic module in already, the job is a lot easier , but the ACC is rediculously expensive from new (and depending on the damage sustained on the used version if it is even useable) , calibration required for the acc sensor is very complicated
your Vin spec will need to be re-written to have the 541 option
and because the sensor is not a retrofit item, ive no idea how the wiring should look for this
Looking into the WDS here may be of assistance
http://www.spaghetticoder.org/bmw/wds/e90/
also calibration
http://tis.spaghetticoder.org/s/view.pl?1/09/09/50
and removal / install
http://tis.spaghetticoder.org/s/view.pl?1/09/09/51