I could be wrong but I remember hearing Todd Howard say that we can customize the Dawnguard Fort or Vampire Fort.

Does anyone now how to do it if it's possible?

I dont think it is.

it was a lie,no upgrading forts

Apparently you can "trick out" these locations the same way you can trick out the plains of Whiterun. Just drop your crap on the floor.

Boom. Tricked out. Just like Todd Howard said.

As you complete the questline, the fort or castle will be slowly upgraded. The addition of the forge when you get Gunmar for example. That's what Todd Howard meant.