I didn't flesh out my level and perk strategy as well as I would have liked (I got it nailed down at level 65). I also assumed the Hard Cap was level 100. My character will still come out, I'd say, approx. 85% the way I would've liked. I'm at level 72 now (gotta get a life, huh :D), and I've mapped out my remaining perks.
Anyhoo, if there are new skills, that will allow me to level up further and pick new perks, which would be awesome. Oh, fyi, I don't like spoilers and I don't want to learn the Dawnguard story (I'm like that with most games), thus my ignorance of Dawnguard.
Best Answer
The answer is no. You cannot raise your level cap beyond the current hard cap which is reached when you max out all base skills to 100.
While Dawnguard introduces 2 new skill trees, Vampire and Werewolf building these trees is self contained and does not contribute to your overall level. You gain perks within these trees through in game actions/achievements, not 'xp'.