Currently there is a 3DFace and AppendFace command. Non of them is really handy if you want to append a face to existing SubD.
There could be one single command that solves the following 2 problems:
-Faces should get appended by default to an existing SubD or Mesh if you start a 3Dface in the Vertex of an existing SubD or Mesh without having to chose a geometry first.
-If you want to append a face to a SubD you can not switch to the control polygon mode by pressing tab. So you can only guess where the control points are in the smooth mode. The control polygon view should be default while appending faces.