Allow fields

Simple Block provides config entities that don't support adding fields.

as a custom solution, you can add a new property via hook_ENTITY_TYPE_alter(&$entity_types) hook.

e.g.

$configs = $entity_types['simple_block']->get('config_export');
$configs[] = 'category';
$entity_types['simple_block']->set('config_export', $configs);

then you can render the field via form alter hook and use it wherever you want
e.g.

$category = $form_state->getFormObject()->getEntity();
$form['category'] = [
  '#title' => t('Category'),
  ...
  '#default_value' => $category->get('category'),
];