I did one on my old trialtechs, put a star nut in the existing steerer, drill it out to 6/7mm.
Weld the extension on from the outside, grind backto flush, but make sure you do a mini route weld.
Bang a starnut in the newly added extension, put a big bolt through from the bottom going through the first starnut (not threading) then passing into the last one (threading)
You should be left with some exposed bolt hanging out the top, bang a topcap on, put a nut on to tighten the headset with a springwasher, grind the bolt back flush to the top of the nut.
You have the both the bolt and the route weld holding her inplace
Worked for me