You won't be doing much fire damage as a Stam DK, because the stamina morphs of DK skills inflict poison damage (which does extra damage against werewolves, just as an aside, but only if they're in werewolf form). Fire damage is a magicka scene.

However, the Fighters' Guild ultimate Dawnbreaker (alongside all other Fighters' Guild skills) does extra damage to vampires & werewolves, is available to all classes, and is stronger when used by stamina characters because it scales off physical damage, which can be boosted in the stamina CP trees.