I agree with Pete, you'll be fine to cut and extend the threads. Just take your time with the die, use lots of cutting oil, and make sure you back relieve the new threads to break the chip every two rotations or so.
As an aside, in my experience in repairing and building all types of forks, butted steerers are thicker at the crown and thinner at the superior end of the tubing.
cheers,
rody