Hi peter, I have an H1b petition approved till Aug 2017 but my visa hasnt been stamped.. can i get my petition extended even without having been travelled to U

Hi peter, I have an H1b petition approved till Aug 2017 but my visa hasnt been stamped.. can i get my petition extended even without having been travelled to US even once? Also, if i have a valid visa but the petition gets expired. am i still eligible to work on the visa or the petition needs to be extended? can another employer get the petition extended for me?