If your swing speed is moderate, one could use a discarded graphite shaft for extension.
Find the spot where one could insert the donor shaft to 4"-6" down the butt end of the shaft to be extended by cutting off the tip section of the donor shaft a little bit at a time. fit and trim, repeat. After the ideal spot is determined built up the thinner part a little bit with masking tape to prevent shifting and rattle, the tight fit at the top will secure the extension if use enough shafting epoxy. leave at least 2-3 " extension after trimming.
Keep the shaft upside down while curing to prevent the extra epoxy running down the inside of the shaft. You want the epoxy to stay between the shafts for holding strength. You can fill in the uneven surface on the outside with small pieces of tape and epoxy once the extension is secured.
I had seen this method failed before by a golfer with 125+ MPH swing speed. but for the most golfers this method will hold for the remainder life of the shaft .