I found some kind of a solution by now.
Create a DynamicMesh with SplineComponent. Get the Vertecies from the spline points. Use “Append Simple Extrude Polygon” Node. Convert the mesh into a volume with “Create new Volume from Mesh”. Change area class.
Now I can use the spline to create my PCG Area (For example forest), wich does change the area into Forest Nav Mesh, wich in return can be used by the corresponding NPC’s