Here's a nice technique to fairly easily lengthen a steerer tube in the home workshop. Of course it'll work for bicycles motorcycles or butting any other tube together. The ideal would be to replace the steerer altogether with the correct length one but that's technically a whole extra level of difficulty.
So here we go step by step...
First up: below - the fork I want to use is on the right and the donor on the left. Before getting busy cutting anything measure exactly how long you need the steerer to be. First cut the steerer on the fork to be used above the crown, doesn't matter too much where as long as you give yourself enough space to weld and file. A plumber's pipe cutter is ideal as it will guarantee a square cut. Next up measure how long you need to add on to the steerer from the donor steerer and then cut that down too.
I had expected the tube wall to be the same size on both forks - it wasn't. Not a big deal, you need to make a sleeve anyway and it isn't a whole load more difficult to make a stepped one as not. It needs to be a good tight fit. Obviously for this a lathe is a must.
Chamfer the edges of each tube so that the weld will fill nicely and enable a strong joint even after filing flat. Push in the sleeve and then push the tubes together. Hold in your vice ready to weld.
Easiest to make sure it is all going together square and true is to tack every ninety degrees before welding fully around the full circumferance.
Here it is all welded up. To file it smooth I held it in my lathe to spin. For a brief time I did contemplate the idea of turning the weld down. Just for fun I turned the motor on, it was predictably a crazy dangerous arm mangler. Don't do it kids!
So yep, good sense prevailed and I turned the chuck manually whilst filing down.
And here it is all smoothed down. Pretty nice, eh? Can't even see the join. You could now knock the sleeve out if you wanted to but I chose to leave it in.
And here are the forks fitted to my Royal Enfield Bullet twin tube project. They are an almost perfect match for the originals.