You can easily do this here by entering the voucher in the box while you are logged in.

Notice: If you already have an active subscription, you cannot activate a voucher. This only works if you don't have an active subscription with us. 

One solution would be to cancel the subscription and redeem it again as soon as it has expired. 

Another solution would be to write to the support, they may also transfer the voucher amount to your current account.