Aside from the insufficient strength of a JB-welded joint, there is the alignment issue BCRider pointed out. Due to the large amount of clearance between the quill and steerer-tube, you will never be able to get the top section axially parallel to the bottom part. Especially when the stem's wedge is tightened. The top portion will be off at an angle, making the bearing-surfaces of the top and bottom headset cups not parallel. This will put ALL of the weight and road-shock forces carry through a couple of bearings rather than be spread out amongst all of them. You'll end up with an indexed headset in no time, not safe for handling.
There are two safe methods I would employ to extend a steerer:
1. unbraze the steerer tube and braze in a longer one.
2. use a precision-milled sleeve to press-fit inside the steerer to join the original steerer with the extension piece. Ideally, this sleeve would have tapered thickness, thick in the middle, thin at the ends (variable ID, fixed OD). Bevel the mating surfaces of the original steerer and extension piece by 45-degrees so you end up with a 90-degree V-notch. Braze or weld the three pieces together. Then spin the steerer in a lathe and mill down the join so that it's uniform to remove stress-risers (and allow the headfork crown-race to slide down).
Both a lot of work compared to getting another fork.