You can try calling them but given they are TAP I'd keep your expectations in check. One way to extend the voucher is to try to book a flight that has a high probability of being cancelled in which case TAP in my experience generates a new voucher with a new 12 month expiration date. You could also book a refundable flight I think Top Executive is and my guess is if you cancel voluntarily they will issue you a new voucher good for 12 months but I haven't tried this. The challenge you will have is that to retain the full value of the voucher you will need to either find a flight in the exact value of your voucher or purchase a flight slightly more expensive and give TAP more money. If the flight is less than the voucher value it remains on the old voucher which will expire. At this point the best plan would probably be to combine both strategies above.