Add metatags import to migration
Finally I came up with a solution and I post it here in case anyone else has the same problem. There are two tables in the legacy site named metatags and  metapages, in which you can find everything needed for importing the metatags. 

So in my prepareRow() in each node migration class I added these rows, and now the metatags from the legacy Drual 6 site has been imported to my Drupal 7 site :-)

$metapages_path = !empty($row-&gt;nid) ? 'node/'.$row-&gt;nid : false;
if($metapages_path){
    $meta_title_pid = Database::getConnection($target='default', $key='legacy')-&gt;query('SELECT f.title, f.pid FROM {metapages} f WHERE f.path = :path',array(':path' =&gt; $metapages_path))-&gt;fetchAssoc();
}
$meta_title = !empty($meta_title_pid['title']) ? $meta_title_pid['title'] . ' | [site:name]' : '';
$this-&gt;addFieldMapping('metatag_title')-&gt;defaultValue($meta_title); 
$meta_pid = !empty($meta_title_pid['pid']) ? $meta_title_pid['pid'] : false;
if($meta_pid){
    $meta_keywords = Database::getConnection($target='default', $key='legacy')-&gt;query('SELECT f.value FROM {metatags} f WHERE f.name = :key AND f.parent_pid = :pid',array(':pid' =&gt; $meta_pid, 'key' =&gt; 'keywords'))-&gt;fetchAssoc();
    $meta_description = Database::getConnection($target='default', $key='legacy')-&gt;query('SELECT f.value FROM {metatags} f WHERE f.name = :key AND f.parent_pid = :pid',array(':pid' =&gt; $meta_pid, 'key' =&gt; 'description'))-&gt;fetchAssoc();      
}
$keywords = !empty($meta_keywords['value']) ? $meta_keywords['value'] : '';
$this-&gt;addFieldMapping('metatag_keywords')-&gt;defaultValue($keywords);    
$description = !empty($meta_description['value']) ? $meta_description['value'] : '';
$this-&gt;addFieldMapping('metatag_description')-&gt;defaultValue($description);