P: Add a control to mitigate halos with masking

Usually with low f stops and strong subject seperation from the background, adding a mask using the select subject works very fast, accurately and easily. However, if there is strong color or light contrast the subject seperation begins to look unreal when changing the mask settings. 

Is there a way to add/ create a type of buffer mask, where you can create an 'edge mask' around your subject and it can be graduated away from the subject? Or possibly, just adding an option to scale the size of the subject selection mask so you can make it slightly larger than the subject or slightly smaller, then can make adjustments from there?