There is a little roll pin that holds the stock body in place. Once removed the stock will slide freely on the extension tube. All you would need to do is drill the holes in the botton of the tube. IIRC, the back half of the tube is solid and not hollow all the way to the end. I believe the other member I was mentioning talked about drilling and tapping holes and using a hex head screw to hold it all in place. But you may be able to salvage the spring and plug from a real adjustable one.
Mine originally came with a "fixed" adjustable. I myself wonder about doing what you are thinking, but I ended buying a new set up.