I don’t quite understand what you are trying to do, but if you want to list all running AppDomains by attaching Visual Studio to the process, see this link:
Jack Goodenough "Enumeration of AppDomains"
In the same vein, if you want to do this AT RUNTIME (without Visual Studio), you probably need to turn on a managed debugger, which allows you to “attach” to another process. Take a look at an example of a managed debugger, this demonstrates a debugger written in C #. Then it will allow you to perform the “extension” as part of the target application process, for example, in an article by Jack Goodenkauf.
CLR Managed Debugger Example (mdbg)