Add an iPod dock to a current system?

Yes if you have a line in somewhere in the current system - unless you are referring to a Sonos iPod dock that can be added wirelessly.

The Sonos Dock that Kumar alludes to is discontinued, but you should be able to pick one up on eBay. The product guide is at the link below if you want to read about it.
http://www.sonos.com/pdfs/productguides/en/dockguide_en.pdf

Sorry, a Sonos dock.

Sorry, a Sonos dock.
I'm not sure I understand your question, but -- yes, you can add a Sonos Dock to an existing Sonos system.

How do you add it? I don't find it in the list of devices to add.

How do you add it? I don't find it in the list of devices to add.
The product guide document I pointed you to indicates that you set it up as you would a 'Player or Sub'. I've never owned a Dock, so I can't comment on whether this is correct.

When you select Player or Sub it brings you to a list which doesn't include a dock.

When you select Player or Sub it brings you to a list which doesn't include a dock.
Google "sonos add dock" and see if any of the search results assist you.

Looks like a bust. I tried all of them.

Hi Musictoo2, I believe you'd pick the "Add a Boost" option and press the one button on it when asked. Also, that you'll need to have one of your Sonos devices wired into the network so that the DOCK can connect and update. If the software is too old, you may need to give us a call on our support line.

Please note that the DOCK is a discontinued product, and has been for quite some time. I can't speak to how well it'll work with the latest software or what functionality it'll have now or going forward.

It works fine with the latest software. Where it has problems is with the devices that will be docked in it; apple devices that don't have the old docking slots in them will need a cable connector that is compatible at the respective ends. I have not tried this, but with an appropriate cable, even iPhone X may work in it, though why one would want it to is questionable.