The way you've gone about it so far is just fine.
The Administration.Account entity is a specialization of the System.User entity, meaning that when you create an Account object it will automatically inherit all attributes from the System.User entity.
You can see this for yourself when looking at the domain models. When you inspect the Administration.Account entity, by default you will see only the following attributes:
Now try to create an Account object in a microflow and you will notice that there are a lot more attributes than three, that you can set for this object. These are the attributes that are available in the System.User entity. If you wanted to, you could even make another entity a specialization of the Administration.Account entity. This new entity would inherit attributes from Administration.Account and System.User.
However you set up your application, logged in users will always be a specialization of System.User at its root.
Check out the Mendix documentation or learning paths if you want to know more about this.
Administration.Account object inherit from users. The main difference here is that the User object cannot be changed in any way, but the Account object is an extension.