How do I alter a database schema defined from another module?
hook_schema_alter() was not used for adding fields to the schema, it was used for reporting alterations to the schema by your module. This was so that functions such as drupal_get_schema() knew the alterations you had made. These functions have all been removed from Drupal 8 however, so there is no need to report your changes in hook_schema_alter().

You can add fields to another schema using:

\Drupal::service('database')-&gt;schema()-&gt;addField()


Documentation: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Database%21Schema.php/function/Schema%3A%3AaddField/8.2.x