The Sonos Sub needs to be connected to the Sonos One in the Sonos app first before you can use the Sub and One together over AirPlay with HomeKit. 

Thanks!

then using HomeKit I will be able to use them in parallel with other rooms’ speakers(not Sonos ones)?

Whatever Sonos speaker the Sub is connected to in the Sonos app (example: Ones + Sub), look at this setup as one zone or room. You will be able to group other AirPlay 2 compatible speakers with the Ones + Sub room just like you can group other Sonos speakers together in the Sonos app. But the Sub is technically only playing the bass from the audio playing from the connected Ones, not the other grouped speakers… if that makes any sense.

Great. Thanks again