Well the Old Norse keyboard has Æ, Ø and Å already.
But they'd probably be in a different place to where they are on a standard Norsk keyboard.
But this keyboard won't replace your default.
When you install it, it will be loaded to your keyboard-layout-list but it shouldn't change your default
unless you ask it to. Then you can use your Norsk keyboard for everything you want, but anytime you want
to write in Old Norse, you usually just have to press Alt and Shift and it will swap to Old norse.
Then just press Alt and Shift to go back to Norsk. Unless you already have other keyboard layouts in your list,
in which case it will just cycle through them.
One point though, since computers don't recognize Old Norse as a language USED with computers,
I've had to script the keyboard as an alternate Norwegian Bokmal keyboard... which means that it has the initials
NO, so you won't be able to tell which keyboard you're using unless you click on your keyboard settings, or start typing. :P
Good luck! I'm happy to help with any keyboard questions, because it's the only keyboard I use, so I'm very used to it by now.
Subject: OLD NORSE KEYBOARD LAYOUT! 
Or will it just add the ON-letters as an alternative, like say if I hold "alt" or "alt gr"? 