It's not currently possible to extend expired vouchers.
However, what you can do is refund the Voucher order to another Voucher:
1. On the Order page, click Refund and choose the Channel you'd like to do the sale through
b) On the payment page, click Add Products or Vouchers
c) This will take you straight to the Products page in Box Office, where you can click Refund to Voucher on the Voucher Product
d) This will add the Voucher item to the basket to balance against the Voucher you're returning. Click Continue
e) Continue to Order Options and select the correct Delivery Method for the refund
f) Continue onto the payment page and as there's no payment to be taken, click Complete Transaction