Is it possible to show a modal view controller in iOS 6 that present my app in App Store?
Yes you can. (iOS6+)
Take a look at SKStoreProductViewController
NSDictionary *appParameters = [NSDictionary dictionaryWithObject:@"533886215"
forKey:SKStoreProductParameterITunesItemIdentifier];
SKStoreProductViewController *productViewController = [[SKStoreProductViewController alloc] init];
[productViewController setDelegate:self];
[productViewController loadProductWithParameters:appParameters
completionBlock:^(BOOL result, NSError *error)
{
}];
[self presentViewController:productViewController
animated:YES
completion:^{
}];