I want to know if it's possible for a custom addin to reach out for custom command line arguments passed to devenv.exe. Is it? I can't seem to find anything on Google. Thanks.
No this is not possible. You can read the command line arguments using Environment.GetCommandLineArgs but the add-in can't extend the set of arguments that can be passed.
In general extensions in Visual Studio read extra information via registry settings or environment variables instead of command line arguments