Hmm. Sounds like we need to get your terminology straight. You mentioned "balance" a couple of times, with a couple of different meanings (I think).
Balancing a wheel/tire is just to make it ride smooth, and (hopefully) allow the tire to wear evenly. It cannot cure a bent wheel or damaged or misaligned suspension.
Your mechanic is correct that there is no adjustment that can be made for the rear wheel ALIGNMENT. Alignment is done by making sure the wheels/tires are not leaning in too much (camber) or pointing in or out too much (toe in or out) using adjustments in the suspension pieces (if available). Most BMW's have no provision for alignment on the rear, and only toe adjustment on the front- without replacing pieces (offset bushings or upper strut bearings, etc).
If you can clarify what exactly the mechanic told you- we can better try to help you.
EDIT- after re-reading the subject line- it looks like the TOE is out of whack in the rear. The only adjustment can be done by installing adjustable trailing arm bushings and having it set by an alignment shop.
Bret