Question Runtime multilple NavMesh areas on a single object

I am trying to procedurally generate a NavMesh in runtime and I am currently using the NavMeshSurface component from an experimental package as it seems to be the only way I can do that.
I am trying to add different NavMesh areas to different places on a single mesh and can't find anything. One idea that I have is maybe if there is a way I can manually input NavMesh vertices and triangles into the NavMesh / NavMeshSurface in runtime but there is nothing that appears to be able to do that. The closest thing I could find to this is the NavMeshTriangulate class but there is nothing I can find that actually uses this class to make a NavMesh.
I would really like some help on this as I have spent multiple afternoons on this and gotten nowhere.