Hello Lumpiluk,
By on top of other post processing effects do you mean a ball that is uninfluenced by the post processing effect. All post processing calculations happen on the same pass. There are ways of assigning information, through Blueprints, to the material by adding the material as a blendable in a post processing volume.
If you would provide a little more information on why you need this particular circle to be drawn in front of a lens flare. I will then look into this issue further.