You would have to create a custom payment gateway and follow Kentico's documentation for creating your own gateway. Also, you will need to have a way of determining if an item is recurring or not, so may have to override a kentico add to cart method, or create your own control for adding items to the shopping cart. Just finished creating one for Paypal, which may be making it to the marketplace soon. Probably wouldn't be very hard to take that and adapt it to use authorize.net.