Give Kingdom of Norway to your vassal and transfer to him all the duchies you want to drift, while keeping their dejure kingdom for yourself.
This will make duchies drift, but your new King of Norway will start desiring other kingdoms.
On the other hand - why bother? When you have an empire - duchies are too small to concern you - kingdoms drift into your empire instead (you can have Germany de jure in Scandinavia Empire...). If you want to have fewer vassals to manage - give your kings a second kingdom(Norway-Denmark, Finalnd-Rus, etc.). Duchies will not drift this way, but kingdoms will. And as an added bonus - your vassal Kings will have to deal with desiring dukes.