If the are auto-discovered, simply select them and put them in an area. If they are added manual to .yaml, you are out of luck. However, you can craft an auto-discovery message yourself, move them out of .yaml, and then add them to an area.
Ok so it only works with auto discovery. Makes sense. I have them in the configuration file so I will decide if I want to move to auto discovery instead.
As Home Assistant is getting more and more user friendly, this should be something you can do in the GUI. I have a lot of mqtt devices/entities by using zigbee2mqtt. None of them are in areas at the moment, would be nice if it could.