Is it possible to retroactively assign an existing Shopify customer to a subscription group in order to charge them recurring fees for products they purchased previously?
No, it is not possible to convert existing customers into a recurring subscription. They will need to purchase a product as a subscription in order to become a subscriber.
A buy now link can be generated in Bold Subscriptions and can be provided to a customer. Once clicked by the customer, they will be directed into the checkout page with a set subscription product.
More details on buy now links can be found here - https://support.boldcommerce.com/hc/en-us/articles/21309670786964-Buy-Now-Links-in-Subscriptions-for-Shopify-Checkout.