Connecting sonos port to existing amp that powers speakers

Consider the Sonos Port like any other device connected to your amplifier. It has one set of RCA out jacks that you cable to a line in on your own amplifier. 

There’s not enough information in your post to tell if a Port would replace a receiver, there are too many other questions. Is this receiver powering to speakers? What kind (impedance)? If so, you’d need a Sonos Amp, most likely. Both the Port and the Amp have a single line in, so if you use your receiver for multiple inputs (as a switch), you’d likely need to get some sort of switch to access those multiple inputs. 
 

So the receiver powers one set of speakers. The rest are powered by two external amps. I have 4 sets of speakers connected to the external amps (2 to each). The receiver (Yamaha RX-V3000) is set at 8 ohms (can be set to 4). I think I have power to the speaker so I guess the port should do the trick connected as an input to the receiver. Correct?

Below are the specs for the 2 identical external amps.

I think so. You can think of the Sonos Port as any other input device, a tape deck, a CD player, etc. It has an output that is line level, just like all of those devices. It is merely another source of a signal, attached to an input on your Yamaha.