Just re-sell the rear rims to someone with an Z4M/E46 M3/E92 and buy new ones. If it's something that money can easily solve, don't make the matters more complicated. You could trim the wheel centers/wheel hubs 2~3mm here and there and it's probably not gonna make enough visual difference or bring the wheels inward as much as you prefer. At the same time you would weaken the intergrity of the parts no matter what and risk having things go wrong. You certainly wouldn't want to deal with wheel vibrations or broken/shattered wheels, etc.