これはメニューコールバックなので、モジュールにhook_menu_alter()を実装して、そのページに対して呼び出されるメニューコールバックを変更できます。
function mymodule_menu_alter(&$items) {
  if (!empty($items['taxonomy/term/%taxonomy_term'])) {
    $items['taxonomy/term/%taxonomy_term']['page callback'] = 'mymodule_term_page';
  }
}
function mymodule_term_page($term) {
  // Build breadcrumb based on the hierarchy of the term.
  $current = (object) array(
    'tid' => $term->tid,
  );
  $breadcrumb = array();
  while ($parents = taxonomy_get_parents($current->tid)) {
    $current = array_shift($parents);
    $breadcrumb[] = l($current->name, 'taxonomy/term/' . $current->tid);
  }
  $breadcrumb[] = l(t('Home'), NULL);
  $breadcrumb = array_reverse($breadcrumb);
  drupal_set_breadcrumb($breadcrumb);
  drupal_add_feed('taxonomy/term/' . $term->tid . '/feed', 'RSS - ' . $term->name);
  $build = array();
  $build['term_heading'] = array(
    'term' => taxonomy_term_view($term, 'full'),
  );
  if ($nids = taxonomy_select_nodes($term->tid, TRUE, variable_get('default_nodes_main', 10))) {
    $nodes = node_load_multiple($nids);
    $build += node_view_multiple($nodes);
    $build['pager'] = array(
      '#theme' => 'pager', 
      '#weight' => 5,
    );
  }
  else {
    $build['no_content'] = array(
      '#prefix' => '<p>', 
      '#markup' => t('There is currently no content classified with this term.'), 
      '#suffix' => '</p>',
    );
  }
  return $build;
}