add_filter( 'aioseo_keywords', 'aioseo_filter_keywords' );

function aioseo_filter_keywords( $keywords ) {

   if ( is_singular() ) {

      $keywords = explode( ',', $keywords );

      array_push( $keywords, 'anotherkeyword' );

      $keywords = implode( ',', $keywords );

   }

   return $keywords ;

}