I just added a platform in an exterior cell in Skyrim and I want my companions and other NPCs to be able to follow me there.

I attempted to manually generate the Navmesh, but it is not flawless. They can barely walk upon, surely because it is not configurated well enough.

So I want to autogenerate the navmesh, but I fear I might destroy some of the already existing Navhmesh.

Can it cause complications in Skyrim's existing Navmesh if I do so? Also, will I need an xmarker ?

http://www.gamesas.com/topic/1502248-navmesh-question/

You'll find answers here: http://www.creationkit.com/Navmesh

Navmesh is a very complicated feature when it comes to Open world. Mostly, unless it's a world of your own making, you'll have to delete and re create pathways on the Skyrim world in order for it to function. And remember those green lines you see. That means the Navmesh can't crossover.

Now it's for an interior, that's another story. In fact, that's the easiest AND less stress strain. Interiors, you just need to min your size space on the generator. Because it won't require a mass amount like the Open World