You really can't use silver braze (solder) to build up metal.
The solder tends to melt and run, which it's intended to do, so it tends to flow off the top of the sight.
You possibly could do it using one of the silver brazes that's made to fill gaps. This doesn't run like the thinner types do. Another problem with brazes is that they tend to form a "bubble" on flat surfaces.
In other words, the middle of the solder will be much thicker than the edges.
This might make it difficult to get the entire surface built up enough so that when it's filed flat it's still thick enough:
http://www.brownells.com/.aspx/pid=25527/Product/DMX_1260_750_NON_FLOWING_SILVER_BRAZE
A much better option would be to have a GOOD welder use Heli-arc or Tig welding to build up the top using a stainless rod.
This would work better, allow much less heat spread through the barrel, and would be real stainless steel on the stainless sight.
It would also be tougher and a closer color match.
As a temporary, not durable expedient, you could use an epoxy metal to build up the sight. This would last a few shots, especially if you drilled a couple of shallow holes on top of the sight to give the epoxy a "lock" on the sight.
Finally, what I've done before is to make a "try sight" from heavy wire about the thickness of the sight.
Bend it into an "L" shape and use heavy tape to tape it to the barrel in front of or just behind the sight.
Paint the original sight with white paint, and paint the try sight with black so you can see it better.
Use that as a sight and file it down until you're targeted.
Then measure the try sight for height.