The way to do this would be to create a new entity in a different module. Select “Account” as the generalization of this new entity. This means that your new entity will already have all the attributes and associations of Account available.
You can then add your own custom attributes and associations and set the right Access Rules.
To use your newly created entity, you'll want to check where you are creating user accounts and replacing the default account entity for your new entity. For instance, if you're currently using an Account_Overview screen to create your accounts, you can change the entity of the data grid there.
You'll also want to change your demo users, which you can do under your project security settings: