Bigbananabike said:============================================================
If its steel using a MIG welder is a isolated heat way of welding(don't have to use wet bags etc to isolate the heat to that area), if its aluminium then the rest of the fork can be in a water bath or have wet bags on it.
The aluminum crown won't melt off with this method but the region surrounding the weld, i.e. the old top of the fork and the bottom of your extension tube, will have their hardness reduced by being brought up to high heat.
When you heat metal till it glows in the dark, it must be hardened again, chief. New word for the day: annealing.
Besides, you have to find a welder which is skilled in doing thin, cromo tubes. You also have to buy the tubes unless you have access to cromo scraps which just happen to be the size you need. The tube needs the same ID as well as OD or you will have a stress concentrator right at your weld. This last paragraph adds up to this: It's probably not going to be worth it even if you realize a small net savings.