The short answer is you can’t -directly do this - right now. Don’t know whether later versions of Roon will provide this feature.
I use my iPhone or iPad as an Roon endpoint and connect to the Homepod stereo pair that way,
I’m not sure about Mac OS Catalina but Mac OS thru Mojave only offered the ability to connect to a stereo pair of HomePods using the built-in Apple Music/ITunes app. The desktop apps of Qobuz or Tidal cannot connect to a stereo pair of HomePods via MacOS. There are workarounds to this using a third party software called AirFoil but it looked pretty cumbersome to set up. If other Mac OS apps can’t exploit this, I don’t know how Roon will.
The HomePod pair and my iPhone/iPad running Tidal/Qobuz/Amazon is a pretty sweet wireless stereo. Maybe not HIFI but I like to listen in the nearfield and other than a mild bass-bump I like the sound. Sometimes I think it’s all I need.
Roon - as far as I’ve read, or haven’t read, hasn’t said when or if this feature is coming.
Enjoy your 'pods. I am. Just not directly from Roon.
Airplay 2 licensing it seems is very strict compared to old airplay which Roon reversed engineered it’s not offically licensed by Apple so ATM would need official license and follow their strict rules to get airplay 2. It maybe something Roon does not want to pay for or go through the hoops to get it.