I sided with the Dawnguard, and after clearing the whole of Volikhar keep I was just wondering If I could do anything with it in terms of using it as a house. Or if there are any mods that can make it into a house or something.

Rumor has it you can use Castle Volkihar as your personal house, as there are many containers that don't respawn, despite Fort Dawnguard.

More info can be found http://www.uesp.net/wiki/Skyrim:Castle_Volkihar.