Adding a new custom field/column to the content admin - php

Question

I've been able to add an element to the bottom of the table. Although I am unsure how you ADD a coloumn into the body of the table?
function seven_form_alter(&$form, &$form_state, $form_id) {
drupal_set_message("Form ID is : " . $form_id);
//get node_admin_content
//$nodeAdmin = drupal_get_form("node_admin_content");
// Add a checkbox to registration form about agreeing to terms of use.
$form['node_admin_content']['poland'] = array(
'#type' => 'checkbox',
'#title' => t("I agree with the website's terms and conditions."),
'#required' => TRUE,
);
}

Answer 1

The table is built by node_admin_nodes() and comes in a nice render array so you can override it:
// Get the header array and add your new column header
$header = $form['admin']['nodes']['#header'];
$header['new_column'] = array('data' => 'New Col Header');
$form['admin']['nodes']['#header'] = $header;
// Get the table rows and add your new column to each.
// The function used to output the table depends on the user permissions
// so you need to check what type of object is being rendered.
if (isset($form['admin']['nodes']['#options'])) {
$row_key = '#options';
}
else {
$row_key = '#rows';
}
$rows = $form['admin']['nodes'][$row_key];
foreach ($rows as $nid => $row) {
$form['admin']['nodes'][$row_key][$nid]['new_column'] = array('data' => 'Text');
}