i use "alt" and then a combination of numbers to make any symbol which is not exist in the keyboard. you must use the numbers on the side of the keyboard (the part that looks like a calculator).
the numbers for the norwegian symbols:
å= alt, 0229
ø= alt, 0248
æ= alt, 0230
for any other symbols, you can ask professor doctor google;)
Isn't it a bit of an overkill? I simply added Norwegian keyboard layout in system preferences. Not like German, for instance, Norwegian layout does not mix up the standard letter positions. Only punctuation marks have changed their location, so I think Norwegian layout is quite easy to adapt to.