I don't do this to save weight. I've used this same technique to extend the length of a steering tube by 10-15mm. 20mm would be stretching it. All you do is use a no-stop copper tube coupling, for 1 inch pipe that has an ID of 1-1/8. Lightly grease the ID of the coupling, slide it over the steering tube and tape it so it doesn't slide down. Fill with JB Weld epoxy and allow to cure.
I did this only once on a LOOK KG461. I rode it many thousands of miles with no problems.
Now you have fuel for more whining.