I own both internal and NICE frame Mystery Ranch packs as well as Kifaru. I wouldn't think you can make Kifaru or any other belt system work as the MR belt lumber is usually sewn in as part of the pack along with the belt webbing and structural side wings. The only belt parts that are removable from MR packs is the lumbar cushion assembly (pads behind kidneys) and the LH & RH Belt pads. The belt pads are what determine size on a MR pack. So, if you have size Small belt pads, you simply need to swap out for Medium Belt pads. Most MR belt pads are interchangeable, so If you can purchase a used set of NICE frame belt pads or some of the older CUSH belt pads (larger and more padded), either will work. What size are your current belt pads? Should have a tag on them. I have some extra belt pads and maybe I can help you out.