I would not mind if he was a part of it in fact I would be glad to see him he was probably the most interesting and intriguing NPC in that game. He might have something to do with the Dawnguard, but what would a vampire have to gain by working with vampire hunters, are these other vampires that much of a threat or he could appear somewhere else in the add on not working for or against the vampires or the Dawnguard.
Well, it's clear that Hassildor has no love for vampire-kind (going as far as to exterminate those uncivilized vampires who near his domains). It could be that his benevolent spirit makes him decide to stop these vampires from becoming too powerful for mortals to handle...or he realizes that no sun means no crops, and no sun means progressively colder temperatures and both of those mean the eventual extinction of man and mer.
The greatest deception of all would be if Janus Hassildor founded, or supported the Dawnguard in his capacity of Count of Skingrad, hiding his vampirism even from the vampire hunters (if anyone could, it's Janus). Perhaps he feels that the only people who could threaten or kill him are other vampires, so he works to exterminate those others.