Yes, you can create a custom base page. In that page, override the InitializeCulture subroutine, and you have complete access to the request object at the point, and can set the culture to anything you want.
Then have all your other pages inherit from that page instead of System.UI.Page.
However, in ASP.NET the parent culture of "nb-NO" is "no" which is the generic norweign culture. Therefore the fallback rules for nb-NO would be: nb-NO -> no -> website default.
I'm afraid I'm not well versed enough in norweign or the relationships of the primary/secondary languages used there to be of much help.