X, in my opinion has a much easier approach to this. You don't mention what desktop environment you use, so I'll explain how to do it in Gnome. The steps in KDE are similar, but not exactly the same.
Go to System > Preferences > Hardware > Keyboard and click on the Layouts tab. Click the "Layout Options..." button. Expand "Compose key position" and choose one (I chose "Right Win-key is compose").
That's it. Now, if you want to type something like "ö", you press Right Win Key, followed by " and then o.
http://en.wikipedia.org/wiki/Compose_key
Edit: Ooops, I think I misunderstood your question. None of the ASCII codes you mention are extended characters. They're all on a standard US keyboard. I just read the "composing" part and thought, "I know how to do that".....