First, all of the groundwork is laid out for all of Tamriel and included with Skyrim(and blocked off by the invisible forcefield).
No, it's not.
What's there outside of Skyrim is a basic landmass with basic heightmaps and even more basic distant land textures and detail. Most of it is incomplete (ie: Only one side of Red Mountain is presnet with a void the other) or -with the exception of a super low detail white gold tower- completely devoid of detail. There's no smooth, high detail ground meshes, no near distance textures and in a lot of places it's not even solid.
And only portions of the surrounding provinces are present, not all of Tamriel. It's basically there so you don't see a void or a featureless landscape beyond the borders should you get high enough, from say the throat of the world or during a particular Daedric quest.
In Oblivion, we had the same thing with surrounding provinces being present and we didn't have DLC that went to any of those.