The Port is usually what is used to connect a receiver to the Sonos system. Usually, a tape out from the receiver to the line in on the Sonos would work. You could then also send the Sonos signal to the receiver by connecting the line out on the Sonos Port to the tape in on the receiver.
That puts the receiver’s signal in to the world of Sonos, and it can then be sent to any Sonos product you have in wifi range.
Given that you’re talking about an “outside” area, you may want to avoid getting a Sonos speaker altogether, and instead replace the Port with a Sonos Amp, which can then drive your own third party outside speakers. Sonos themselves don’t make outside speakers, but they do have a co-marketing agreement with a company called Sonance, although the Amp would power most any outside speaker on the market, making sure you match power and impedance. That way, you can keep the Amp inside out of the weather, and have the speakers outside without worry.