Presumably you could use the JSObject called from C# to create one although I don't know how much of the process has been documented. And as of the 9.0 SDK you where not able to change the Portfolio to use a custom navigator ( although this may have changed).
Let us know if you need more information.
Thanks, I just found the JSObject example in the 9.1 SDK and am trying that out.. I hadn't seen the c++ API to create a portfolio though. Can you send me a url for that documentation?