It's not currently possible to extend expired vouchers. If you'd like the customer to still use a voucher, then the options would be:
Refund the existing order to a credit voucher - this would mean they would receive a new voucher, worth the same amount.
Or if you didn't want to 'refund' the existing order, then sell a new gift voucher in Box Office using a Custom Payment Method (you could always call this Payment Method something like 'expired voucher payment'), so that you're not charging the customer anything.