My Cannondale has a threaded fork, so I had installed a threadless stem adapter so I can install threadless stem. However, to make things more comfortable on longer rides, I need to raise my handlebars and wondering how I can do that.

Is there a way to extend the threadless stem adapter or perhaps to buy one that's longer than the standard one?

I recently bought one for one of my bikes, and it was way too long so I had to cut it down. I think it probably stuck >6 inches out at it's lowest setting, so I think that you'll be able to find a long one. I bought mine at a local bike store so I don't know the model or anything.
Incase you don't know, you should be able to raise the stem somewhat, by loosening the top bolt and just lifting the handlebars up. There should probably be a minimum insertion line you don't want to go past though

You mean pull out the threadless stem adapter a bit to extend it? That's what I was trying to do yesterday, however, considering how long ago I had installed that I had forgotten that it was possible to pull it out. And it must have gotten stuck there, so I probably need to try again and jiggle it a bit. I will make sure the minimum insertion point is inside the steerer.