@Justin: Probably the cast exception you received is: "Unable to cast object of type 'PBObjLib.SymbolClass' to type 'PBSymLib.PBControl'."
You must know that PI ProcessBook has been built with COM and not .NET. Some .NET interop libraries were written to allow their usage within the .NET world but not everything is accessible.
The Add method does not return the exact same object type if you are on the COM-based world or .NET. On the COM-based world, the casting of the returned object into one of its extended interface works flawlessly but on the .NET side, a different in the definition makes that casting not possible.
One possible way to solve your problem would be to write a C++/CLI library to perform that operation for you and return the PBControl object to your .NET code that would incorporate this aforementioned library.
I hope this helped!
@Mathieu: Thank you for the response.
I will try adding a C++ library and post my results.