Yes, I have! I flatten all manner of stones on a DMT Coarse or DMT Extra Coarse diamond stone. In truth you shouldn't have to flatten a Norton flattening stone very often but I, as always, recommend checking the stone with a straightedge every so often. I use a machinist's rule for this as they are precision straightedge tools made by Starrett. A good thing to remember is that "flat" is a specification and not a state of perfection. "Flat" is just a question of money as any metrologist will tell you.