I think the only way is to play music on the iOS device and have that play through Sonos either via a bluetooth connection or via a wired to Sonos line in jacks. Both depend on having Sonos devices that have this ability to receive music from the device. And it may need a iOS device to be dedicated for this purpose, connected to mains power as well. 

I don’t like operating in this mode, but sending music to the SONOS system via AirPlay 2 or Bluetooth will take advantage of any Spotify Volume normalization.

The only stable long term way to do this is via wired line in; hence a dedicated oldish iOS device is recommended. And since it will be playing all the time, it is best left attached to mains power. If the Spotify normalisation feature really does work, this is the way to get playlists playing without having to fiddle with sound levels too often. Or do what I have done which is delete tracks that are sound level outliers from the playlist itself.

And it isn't just Sonos that misses out this important feature. I usually use Echo devices as the front end, and they don’t normalise sound levels either.