Hi Norbert,
If you are trying to draw a cuboid which has irregular z values and don't think that extrusion will work, another option could just be to draw a series of vertical polygons for the faces of the shape. Here's some example code that draws the sides of a 3d shape with some variation in the z values. Note you could probably do this in a more elegant fashion using a multi part polygon.
SimpleLineSymbol* outlineSym = new SimpleLineSymbol(SimpleLineSymbolStyle::Solid, Qt::black, 4, this);
SimpleFillSymbol* fillSym = new SimpleFillSymbol(SimpleFillSymbolStyle::Solid, Qt::red, outlineSym, this);
PolygonBuilder* bldr = new PolygonBuilder(m_sceneView->spatialReference(), this);
bldr->addPoint(Point(0., 0., 40000));
bldr->addPoint(Point(0., 1., 50000));
bldr->addPoint(Point(0., 1., 100000));
bldr->addPoint(Point(0., 0., 100000));
Graphic* g1 = new Graphic(bldr->toPolygon(), fillSym, this);
PolygonBuilder* bldr2 = new PolygonBuilder(m_sceneView->spatialReference(), this);
bldr2->addPoint(Point(0., 1., 50000));
bldr2->addPoint(Point(1., 1., 50000));
bldr2->addPoint(Point(1., 1., 100000));
bldr2->addPoint(Point(0., 1., 100000));
Graphic* g2 = new Graphic(bldr2->toPolygon(), fillSym, this);
PolygonBuilder* bldr3 = new PolygonBuilder(m_sceneView->spatialReference(), this);
bldr3->addPoint(Point(1., 1., 50000));
bldr3->addPoint(Point(1., 0., 50000));
bldr3->addPoint(Point(1., 0., 100000));
bldr3->addPoint(Point(1., 1., 100000));
Graphic* g3 = new Graphic(bldr3->toPolygon(), fillSym, this);
PolygonBuilder* bldr4 = new PolygonBuilder(m_sceneView->spatialReference(), this);
bldr4->addPoint(Point(1., 0., 50000));
bldr4->addPoint(Point(0., 0., 40000));
bldr4->addPoint(Point(0., 0., 100000));
bldr4->addPoint(Point(1., 0., 100000));
Graphic* g4 = new Graphic(bldr4->toPolygon(), fillSym, this);
m_overlay->graphics()->append(g1);
m_overlay->graphics()->append(g2);
m_overlay->graphics()->append(g3);
m_overlay->graphics()->append(g4);
Does that help with your problem?
Luke