What is the ruling on a reciter extending sounds in his recitation of the Quran in places where there are no rules of extension (madd), with the intention of tartil?

A madd letter that is not followed by one of the three causes of madd (lengthening) is only extended by the original natural madd. Any extension beyond the original madd is not permissible and is considered an error (lahn), whether in a tartil (recitational) state or otherwise.