In a clean verbatim file, you can add words that are not heard in the audio to ensure the file is grammatically correct.

 No, you do not interpret, you transcribe. It is possible to insert unheard or perhaps unspoken words by use of parentheses, but it is generally not advised. it somewhat depends in the purpose of the transcript. the use of "(sic)" to indicate that what has been written is was what was truly and really said despite its obvious error or errors is also used.