Simply stated you need to get an audio signal to a subwoofer. AFAIK, with a Sonos subwoofer it appears the only way to do this is wirelessly through the Sonos ecosystem. So you can't use the Sonos sub with WiiM.
With other subwoofers (RSL, Klipsch, SVS, etc.) you'll feed the audio signal to the RCA input on the subwoofer either wirelessy using multiple WiiM units or with a cable between the WiiM and the subwoofer.
To connect to the subwoofer wirelessly you will use a second WiiM unit as the wireless receiver for the subwoofer. You will connect the output of the second WiiM to the input on the subwoofer. You will use the multi-room audio to send the music to the main speakers and the subwoofer simultaneosuly.
If you want to connect a WiiM to the subwoofer with a wired connection then you do it in one of two ways. The WiiM Ultra and WiiM Amp have subwoofer outputs built in. With these WiiMs, you can simply connect an RCA cable from the WiiM unit SUB OUT to the LINE IN on the subwoofer.
If you are using a WiiM Mini, Pro, or Pro Plus, you can possibly use an audio cable Y splitter to take the output of the WiiM and feed it to both the main speakers and subwoofer simultaneously. Before attempting this you will want to make sure that feeding the output to two different places simultaneously will not create a problem for any of your equipment.