If you don't allow users to login, register, or access frontend then they've no way of purchasing a subscription. If you give them subscriptions manually from backend then it bypasses payment process and you won't have a way of letting CBSubs handle the subscriptions automatically. CBSubs requires a user to be registered to make a purchase.
You can use the CBSubs AcyMailing integration combined with the AcyMailing extension to subscribe users to yearly newsletters then it can remove them when their subscription deactivates.