Pretty sure Extended Warranty does not cover wheels. You can read the policy
here and on the pages describing what's covered, wheels are not mentioned and there's a clause at the end that states "Any components not specifically listed are not covered.". I assume replacement alloys provided under the original warranty would only carry the standard replacement parts warranty. Arguably genuine BMW replacement parts (where fitted by a BMW technician) have a two year warranty from the date of fitting, but I wouldn't be surprised if a claim for wheels falls on deaf ears.
I had all my wheels (442Ms) replaced just before my new car 3 year warranty was up. 1½ years later and the replacements are already rotting. When they get bad enough I'll either replace them with non-diamond cut OEM wheels in the hopes those last longer or I'll go after market (VMR or similar).