You can import C# source code by using this C# Reverse Plug-in (free).
Astah can generate C# Skeleton Code from selected Diagram Elements.
The Definitions of Classes and Operations are exported as Documentation Comments.
1. Go to [Tools] – [C#] – [Export C#] in the Main Menu.
2. Select the target folder, target Model, from which .cs files are to be generated and click [OK]. Specify the charset to use upon exporting C#.