As to what I know you can not extend the voucher validity. But its always good to first check with the insitute that sold it. If I were you I will try to finish the exam before the deadline. You have more than a week to do it. So good luck.
For part II, it does not have an expiry date. But as the old exam has expiried, you need to submit your assignment and finish part III exam by end of this year. So, I think you can continue to finish you part II assignment first, and then buy a new voucher for part III.