Add text field to poll entity
Poll is not a fieldable entity, so this is not possible in ui.

You can do this in code:

use Drupal\Core\Entity\EntityTypeInterface;
use Drupal\Core\Field\BaseFieldDefinition;
/**
 * Implements hook_entity_base_field_info().
 */
function mymodule_entity_base_field_info(EntityTypeInterface $entity_type) {
  if ($entity_type-&gt;id() === 'poll') {
    $fields['description'] = BaseFieldDefinition::create('text')
      -&gt;setLabel(t('New discription field'))
      -&gt;setTranslatable(TRUE);
    return $fields;
  }
}


Apply the schema update on the database:

drush entity-updates