It is not possible to directly create custom profile fields to be used for internal purposes that will only display on the admin > site users page but not show on the create account form or in the customers profile area of the storefront when they log in.
But, with a few site limitations it is possible to accomplish this with a workaround.
- Create your custom profile field and enable it for the site
- Do not allow self sign-ups for the site
- Do not give your site users permission to "edit their own personal account info"
In this scenario, the custom profile field will be enabled for the sign up form, in the admin > site user area, and in the site users profile area of the storefront, but you will be hiding those pages/forms from your customers so they will not see them.
For more information on these areas, refer to the following documentation pages: