On the right side of the Airtime interface, click Webstreams tab, then click +New button. In the window on the right, fill in the information for the stream that you'd like to rebroadcast. Once finished press the Save button. You can now schedule that Webstream to any show in Airtime Pro. Once a show with a Webstream starts, Airtime Pro will connect to and rebroadcast the stream to your listeners.
Read more about Webstreams here!