I think your question is more about how to "change" users instead of where to store the admin user details, right?
In the Core Service API it's possible to directly impersonate other users, but there is no TOM.NET counterpart that I'm aware of.
In TOM.NET (the Event handler code), you need to create a new instance of the
Session class for the admin user. Take a look at the API documentation, namely the
Session class and its overloaded constructor which takes in an account name. Also take note of the Remarks section about the current user needing to be configured as an impersonation user. You can search the SDL docs or here on SE about what these users are and how to configure them.