Replace the hoses, do not use an additive to attempt to seal any leak. The job is pretty straight forward. Since the system will drain when you replace the leaking hose I would replace the other hoses while you are at it. When you refill the system, fill the reservoir and then start the engine and turn the wheel lock to lock a few times. Then shut it off and add more fluid. Do that until the reservoir stays at the full mark and all the air is out of the system. Make sure you use the BMW power steering fluid or a fluid with the same specifications. The following link may be helpful:
Power Steering Pump - DIY - E46Fanatics