You can either use Custom Meta module or roll your own module:
From Custom Meta project page:
The module allows you to define and manage custom meta tags. Simply
select the Meta attribute (property, name, http-equiv) and set the
value and content value in the format and the module will add your
meta tags to all non-admin content for your site.
An example of writing your own module:
my_metatag.info
name = My Metatags
description = Provides my custom Metatags.
core = 7.x
version = 7.x-1.x
dependencies[] = metatag
files[] = my_metatags.metatag.inc
my_metatag.module
<?php
/**
* Implements hook_ctools_plugin_api().
*/
function my_metatags_ctools_plugin_api($owner, $api) {
if ($owner == 'metatag' && $api == 'metatag') {
return array('version' => 1);
}
}
my_metatag.metatag.inc
<?php
//
// Implements hook_metatag_info().
//
function my_metatags_metatag_info() {
$info['groups']['my_metatags'] = array(
'label' => t('My Custom Metatags'),
);
$info['tags']['my_custom_metatag'] = array(
'label' => t('My Custom Meta Tag'),
'description' => t('This is a custom meta tag'),
'class' => 'DrupalTextMetaTag',
'group' => 'my_metatags',
);
return $info;
}
//
// Implements hook_metatag_config_default_alter().
//
function my_metatags_metatag_config_default_alter(array &$configs) {
foreach ($configs as &$config) {
switch ($config->instance) {
case 'global':
$config->config += array();
break;
case 'global:frontpage':
$config->config += array();
break;
case 'node':
$config->config += array(
'my_custom_metatag' => array('value' => 'This is a default value.'),
);
break;
case 'taxonomy_term':
$config->config += array();
break;
case 'user':
$config->config += array( );
break;
}
}
}
Hope that helps.