이렇게 하믄 됨 ㅋㄷ;

ExistingAccountViewController *viewController = [[ExistingAccountViewController alloc] initWithNibName:NSStringFromClass([ExistingAccountViewController class]) bundle:nil];

CATransition *transition = [CATransition animation];
transition.duration = 0.5;
transition.type = kCATransitionFade;
transition.subtype = kCATransitionFromTop;

[self.navigationController pushViewController:viewController animated:YES];
[self.navigationController.view.layer addAnimation:transition forKey:kCATransition];