I don't think there's a way to add other application functionality to the Connect system, but as bmninada mentioned, you could just use Bluetooth and pull up the iHeart radio app.
That being said, does Mazda ever update the software on their infotainment systems so it would support more/new/different apps or add functionality to the additional ones? I actually use Stitcher and the absence of a forward 30 seconds button on the Connect system is a real bummer, so I stick to the normal bluetooth/use the app functionality.