Can a medpor jaw implant be added to the top of an existing implant or will I need to remove the existing implant?

Generally one can "add" overlying material to an implant that is well fixated. One can use another material as well that is easier to 'carve' and customize at surgery to prevent over doing or excessive contours if you are only using a 'small, medium, large' option.
Many surgeons do remove them however sp speak to your surgeon