Sure it's possible, but I would advise against it. Apple is moving away from buttons with rounded rectangle borders. Any place you use them is going to look out of place in an iOS 7 app, and you will start to get dinged for it.
As for why iTunes Radio still has a rounded button, I suspect 1 of 2 things: It's a vestige that they haven't updated yet, or there is a holdout for the old look in the iTunes team. If there is a holdout on the iTunes Radio team that likes the old style, they will likely get smacked around and forced to update to follow the new UI Guidelines.
All that being said, what you want to do is quite easy and you are on the right track.
You need to create a custom subclass of UIButton, and set your button(s) to use that class. Then set your button type to "custom" in IB.
Finally, add setup code in your custom button class that does this:
Code:
self.layer.borderWidth = 1;
self.layer.borderColor = [UIColor lightGrayColor].CGColor;
self.layer.cornerRadius = 8;
self.layer.masksToBounds = YES;
The last line, setting masksToBounds to YES, isn't strictly necessary unless you are going to fill the button with a color (which you will probably want to do).