You can add an extra field to a Drupal entity via hook like it is described here: https://gorannikolovski.com/blog/display-and-form-pseudo-fields-drupal-8...

There is one caveat to this explanation. If you use field groups to group such extra fields you need to put them into a container (at least for the form extra fields), otherwise they will not show inside the field group.

So instead of this