A reader writes that according to KLM's web site you can use miles to upgrade from fare class U. If you book, however, you can only choose between economy and business class. Thus, you cannot book a particular fare class.
You can do so at KLM's American partner Delta Air Lines, so he books KLM-flights at Delta. The U fare class is considerably more expensive than the lowest fare, but he does book it to be able to buy an upgrade with miles.