Custom fields

@solarDog

Yes, you can use the hook_form_alter() and add your additional form field. Replace the default submit handler with your custom submit. Write the logic to handle your new fields into it.

Example:

/**
 * Implements hook_form_alter().
 */
function MODULE_NAME_form_alter(array &$form, FormStateInterface $form_state, $form_id) {
  if ($form_id == 'webform_email_reply_form') {
    $form['new_field'] = [
      '#type' => 'textfield',
      '#title' => 'First name',
      '#required' => TRUE,
    ];

    // Custom submit handler.
    $form['#submit'][] = 'MODULE_NAME_webform_email_reply_form_submit';
  }
}

/**
 * Custom submit handler for webform_email_reply_form.
 */
function MODULE_NAME_webform_email_reply_form_submit($form, FormStateInterface $form_state) {
  $new_field = $form_state->getValue('new_field');
  // Do the Other logic.
}