NSMutableArray *ary = (NSMutableArray *)[NSArray arraywithObjects:@"ABC",@"DEF",@"GHI",nil];

NSArray *ary;
ary = [NSArray arraywithObjects:@"ABC",@"DEF",@"GHI",nil];
NSMutableArray *mutableAry = (NSMutableArray *)ary;
とでもしているんだろ