I'm still struggling on this issue after our other requirements are finished. I found we can use GeometrySink alike classes to implement the outline effect; but I'm not familiar with c++; see this article:
<http://msdn.microsoft.com/en-us/library/dd317121.aspx>

> 
> More complex shapes can be created by using the ID2D1GeometrySink
>  interface to specify a series of figures composed of lines, curves,
>  and arcs. The ID2D1GeometrySink is passed to the Open method of an
>  ID2D1PathGeometry to generate a complex geometry.
>  ID2D1SimplifiedGeometrySink can also be used with the DirectWrite API
>  to extract path outlines of formatted text for artistic rendering.
> 
> 
> 

If you have any suggestions or ideas, please let me know.

Best regards,
Howard