You can use the Fog primitive and you can have several different fog primitives in the scene,. all with different settings, and you can use the Scale tool to make large flat fog “Box” or make a “Sphere” shaped fog, and then scale it in the scene.
You could also use the Surface replicator to distribute fog primitives in the scene, and to vary the size and rotation of them ,.
or, use the “Distance fog” in the scene atmosphere section.
but you can also use the “Volumetric cloud” to create a fog effect,. you’ll need to scale it down to fit your scene.
Finally, and the quickest and easiest method .... Post production.
Use an image editor to composite a Fog or Volumetric cloud rendered out with Alpha channel, and adjust the opacity as required.
Hope it helps