Dave is likely onto something. Assuming your drain pipe comes out of the dam close to horizontal, you could add a 90 degree elbow with the open end pointing up towards the sky. That would give you about a 12 inch rise in water depth. At that point, you could add a short stub of straight pipe to whatever height you desire. PVC pipe comes in a variety of flavors and is "usually" color coded. I would measure the outside diameter of your drain pipe (assuming that it is NOT the flared end that is exposed) and then match it to what your local hardware store has and go from there. If it is the flared end, you could cut it off or add a short non-flared stub. I do not believe you'll find adjustable sections at your hardware store, that would be something rather odd and most likely expensive. If you really think you need some adjustability, you could add the elbow then a short stub, then a "slip-on" to threaded coupling or even a union. This would allow you to unscrew your final short stub (so long as it has the same "slip-on" treaded coupling) and change it without cutting it apart and starting over.

Keep in mind that I am not a pond pipe expert, just someone that thinks he can do anything. You could easily rack up 100 to $200 in pieces.

I have to say that raising the level of a pond reduces the freeboard (the dam height above your drain pipe collection). This reduces the amount of water your pond can take during a larger rain event before it goes over your emergency overflow. Just keep that in mind.

Fish on!,
Noel