Combine multiple columns into a single column

I am trying to combine multiple columns in a long admin table of a module (user_mailman_register) into a single column. The module calls theme('table', $headers, $items) for creating the wide table. How can I do this correctly using the theming system?

  $headers = array(
    array('data' => NULL),
    array('data' => t('ID'), 'field' => 'lid', 'sort' => 'asc'),
    array('data' => t('Name'), 'field' => 'name'),
    array('data' => t('Web administration'), 'field' => 'webadmin'),
    array('data' => t('User invite'), 'field' => 'user_invite'),
    array('data' => t('User notify'), 'field' => 'user_notify'),
    array('data' => t('List admins notify'), 'field' => 'user_admin_notify'),
    array('data' => t('Allow unsubscribe'), 'field' => 'allow_unsubscribe'),
    array('data' => t('Allow temporary disable'), 'field' => 'allow_temp_disable'),
    array('data' => t('Allow digest'), 'field' => 'allow_digest'),
    array('data' => t('Description'), 'field' => 'description'),
  );

  $query = 'SELECT * FROM {mailman_lists}'. tablesort_sql($headers);
  $num_per_page = 15;
  $result = pager_query($query, $num_per_page);
  $lists = array();
  while ($list = db_fetch_array($result)) {
    $cmd = 'Edit';
    if (!_user_mailman_register_list_allowed($list)) {
      $cmd = 'Activate';
    }
    unset($list['web']);
    unset($list['webarch']);
    unset($list['admin']);
    unset($list['webpass']);
    unset($list['command']);
    $list['manage'] = l(t('Manage'), 'admin/settings/mailman_manager/edit/'. $list['lid']);
    $lists[] = array_merge(array('status' => l(t('@cmd', array('@cmd' => $cmd)), 'admin/settings/user_mailman_register/edit/'. $list['lid'])), $list);
  }

From what I can make out of this, Can I combine the strings from the different cells and unset the cells that have been combined like this

$list['name'] = $list['name'] . '<br />' . $list['webadmin'] . '<br />' . $list['description'];
unset($list['webadmin']);
unset($list['description']);

and then taking out the corresponding items from the headers array.
I suspect this is not the right way as I have to put raw HTML strings here. What is the right way to do this through themes? Can somebody point me to the right direction. It would be great if I can see some examples.