Hi!
I was wondering if navmeshes in skyrim can be set by for example scripts. I mean, let's say that I have a wall, which later dissapears. I want NPC to, while wandering, "see" the wall, and after it disapears, be aware of that and go to places previously blocked by it. Is it possible?

Yes. What you'd want to do is build your navmesh assuming the wall doesn't exist. Place a collision box on the ground next to the box. Set it to use the wall as an enable parent. Then size the collision box to fit the dimensions of your wall. On the "primitive" tab for the box, set its collision type to L_NAVCUT. NPCs will navigate around the wall while it's there, and will walk through it when it's not.