Having milled a pocket in the normal way, I would like to mill a small taper on all sides of the pocket, ie around the inside of the frame. It can be a series of small steps.  Does this exist at present?

Usually, people buy tapered end mills for such things. They can get expensive. Most cam programs can perform such tasks with regular end mills, router bits, or whatever. Of course, your final results regarding the finish quality will usually suffer.