Unfortunately, you can't. These vouchers are only valid for a specific period; after that time ends, they are not usable. I suggest you use them, as soon as possible, even if people fail - you get 5 attempts a year, so can consume other vouchers you may find or pay for other attempts, and the report gives you a very high level breakdown of some of the areas that people may need to do more research on - and who knows you may pass!
I would investigate whether your organisation is part of the Enterprise Skills initiative as well - if it is, then you can get 50% exam. You will know when you attempt to sign in with your work account.