This may be a stupid question, but I'm not familiar with this kind of stuff. I have Skyrim for my PC, I wanted to purchase the Dawnguard add-on for my pc, I did not purchase the game through steam, can still I purchase the DLC through steam and access it? If not, how would I go about getting the DLC? Any help would be appreciated.

Just purchase the DLC through Steam. Skyrim is tied into Steam and your Steam account regardless of where it was bought, so all the installation of the DLC will be handled for you.