Economy Extra is a completely different product than the Eco+ Seating on UA, it is more a class between Economy and business - not just a slightly better seat. I think right now it is not possible to book Economy Extra award space on SAS (reason unknown), only economy or business.

Anyway, as far as I know it is not possible to upgrade non-revenue tickets by any means, be it miles or via cash.

This is true for SAS EuroBonus at least, not sure if MilePlus will give an option to buy Plus seats on SAS or upgrade a economy award ticket but I'm pretty sure you need to e prepared for a trip in the back of the bus.