Awe heck....must have had a couple too many brews last night....just realized that what I previously posted is not accurate.....the pressure on a cap placed on the outlet would only be the difference between the elevation of the water surface level in the pond and the elevation of the cap on the outlet X the density of water. So, if you are wanting to raise the level in the pond by say, 5 ft, then the pressure on the cap would only be 5 ft X 62.4 pcf = 312 pounds per square foot (2 psi or a total of about 250 pounds of force). The pressure at the bottom of the pipe structure, however, would still be the full difference between the elevation of the water level in the pond and the elevation of the bottom of outlet structure (horizontal portion going through the dam) as I posted yesterday, so leaks developing at any of the joints could still possibly be an issue.

Sorry for any confusion I may have caused...

Still think that, barring the freezing issue that Tim mentioned, a simple extension is your best bet. I would be concerned about a hard freeze busting that pipe, but if it has lasted this long, maybe the steel in the existing pipe is thick enough to resist the expansion pressure from freezing water. You'd definitely want to make sure the extension is equally as strong at the existing pipe...