@Richard_Bailey: You can install software like AirServer on your Mac – this will turn your iMac into an AirPlay receiver and make it visible as an AirPlay zone in Roon (Gaylord is my MacBook Pro - don’t ask ;-)):
Once enabled, you can group the AirServer/iMac zone with your Apple TV (you may want to disable the Roon/RAAT zone for the iMac to avoid confusion – or name it differently, say ‘iMac-Roon’ and ‘iMac-Airplay’).
Not sure how well the AirServer zone performs when grouped with your ATV over Airplay, but I guess you will be fine. AirServer has a 14 day trial for you to find out. 