Nav Mesh - instead of Nav Mesh Modifier Volume - assign meshes as not walkable

Dear community,

I am new to unreal.
Would like to have the selected walls (Screenshots, as attachment) as not walkable Meshes.
I know there is a nav mesh modifier volume which I could place to avoid this areas, however it is at more complex geometry really a lot of work to place them.

Is there a simple solution to assign those meshes as not walkable (at unity it had been possible)…???

another thing which is strange… as the geometry goes down in Z, underneath the ground plane, it displays the nav mesh area at the whole ground plane.

I would be super happy if someone could help me.
Thanks in advance