I got it!
In functions.php intercepts the 'language_attributes' filter and make a substring
function add_custom_opengraph_namespace( $input ) {
return substr($input, 0,-1) . ' customns: http://ogp.me/ns/fb/customns# " ';
}
add_filter( 'language_attributes','add_custom_opengraph_namespace');That's it!