You could write a custom config parser and add a concat method :D
your config could look like this:
[globals > Config->parse]
; overwrite
UI = theme2/
; extend / concat
AUTOLOAD <= foo/
and the parser like this:
class Config extends \Prefab {
   /** @var static \Base */
   protected $f3;
   function __construct() {
      $this->f3 = \Base::instance();
   }
   public function parse($key, $value, $scope) {
      if (is_int(strpos($key,'<')))
         $this->f3->concat(rtrim($key,' <'), ','.$value);
      else
         $this->f3->set($key, $value);
   }
}tested, works, but no warranty at all ;)