I have a 4A pond and I want to raise the water level about 8". I have the type of system that drains the bottom water over a 12" PVC drain pipe. Is the best solution to attach another piece of PVC? And, how is the best way to attach the PVC. The dam and spillway can accommodate the additional water.

Sounds like a coupling would work.
You will very likely have to remove the outer pipe, then just slip a coupling over the standing pipe & re-install the outer pipe.

Ric, Glue the 2 PVCs together and use a rubber "sleeve"? If I can't find a 12" sleeve, is there another way to hold the two pieces together?

Most PVC pipe that size I've seen is sealed & heeled together by a rubber gasket built into the female end. (you grease it before slipping it on & seating it in place with the help of 2x4 or sledge hammer) You can drill some screws though the sides that overlap then back them out half way & coat with a good silcone clauk then rescrew back in to seal them. Hope this make sense & helps.

Thanks Will, Think I'll go that route.