Hi
Based on my knowledge, COM Addin works commonly based on the the Word's
Object Modal. And the C# Addin is just the managed Wrap for legacy C++/VB6
Com addin.
Since the Word's Object Modal did not expose the Task Pane for us, it is
hard to manipulate via the Object Modal.
So from VSTO v2(Whidbey), we have a managed interface to add the winform
control into our own task pane.
Here is a link for your reference.
http://blogs.msdn.com/vsto2/archive/2004/03/11/88183.aspx