1. Are these additional outdoor speakers self powered? If so, you’d need a Sonos Port to provide them a Sonos signal. If not, you’d need a Sonos Amp to provide them both power and a signal.
2. This would be integrated in to you existing system as a new ‘room’ in the Sonos software. Note that when ‘grouped’ with any of the rooms playing a TV’s input, the sound would be slightly delayed, but when streaming music, would be completely in sync.
3. Depends on 1. If they’re self powered, yes, they’d need to be plugged in, and you’d still need to pride them the audio signal with another wire. If they’re powered by a Sonos Amp, both of those would be combined in a single wire, much like in a standard speaker setup with a receiver. 
4. No. Only Sonos self powered speaker would do so, not any third party ‘outside’ speakers. Even the outdoor series from Sonance, a marketing partner with Sonos, requires wiring with a Sonos Amp. I don’t know that for permanent outdoor use, there are any ‘wireless’ solutions. 

Thanks for the quick reply!  

Just to clarify, the outdoor speakers would be the Sonos outdoor speakers.  So...am I correct in understanding you that the Sonos speakers would still require an amp and need to be hard-wired to that amp? 

If the amp needs to be indoors, how would we hardwire the outdoor Sonos speakers to the amp which is indoors?

Sonos doesn’t ‘make’ outdoor speakers, but they do sell Sonance outdoor speakers, which would need to be powered by a Sonos Amp.

Generally speaking, you would need to run the appropriate speaker wires from the speaker posts on the Amp to the speakers themselves. I’m not sure I would know ‘how’ you would need to run those wires for you. 

You could hire a contractor to run the wires. It would be best to run the wires inside “conduit” (a pipe), but you can purchase “direct burial” wire that could be run overland or underground in a shallow trench. In any case pay attention to where the wire runs so that you can avoid damaging the wire in the future. (perhaps slicing into the wire as you plant flowers or such.)

Thank you, Airgetlam and Buzz!  Your suggestions are really helpful.  I think hiring a professional and running a conduit is the best solution.  And thanks fore clearing up the confusion re: the outdoor speakers not being Sonos speakers.

It’s harder than average, since Sonance makes some speakers that have Sonos embossed/printed on them, basically due their partnership, but the speakers are made by Sonance, and (also) sold on the Sonos website, but they really aren’t Sonos, and not self powered. 

Got it!  Will likely buy a couple of Roam speakers and go that route.  Though they need to be charged, it’ll be the easiest solution for us.  Thanks for all of your help and great suggestions!

Just as a note, the Roam is not designed as a permanent outdoor speaker. It certainly can be used outside, but I wouldn’t leave it there permanently. 
 

Also, make sure you have a good WiFi signal where you plan on placing the Roams outside.  Roams can be used with bluetooth only, but they cannot be a stereo pair without WiFi, or play in sync (group) with your other Sonos speakers.  From my experience, the Roam and Move have pretty good WiFi radios in them and can connect farther away from the router than other devices typically would, but you still need to have a signal out there.