Ok, i found a way how to easily convert them, but i dont know wether this is a good way or not:

private MeshVisual3D GetMeshVisual3D(MeshGeometry3D toTransform)
        {
            MeshVisual3D returnMesh = new MeshVisual3D();
            Mesh3D mesh;
            if (toTransform != null)
                mesh = new Mesh3D(toTransform.Positions, toTransform.TriangleIndices);
            else
                mesh = new Mesh3D(point3Ds, triangles);
            returnMesh.Mesh = mesh;
            return returnMesh;
        }

now my problem ist to get the slice thats created with the cutting (the 2D-View of the top-view from the cut) i know how i get the single points of that plane (with the MeshGeometryHelper.GetContourSegments), but unfortunatly they are not connected. Is there a way to get them connected?