Can I turn a one-time order into a subscription? (Essential Subscriptions app)

No. An order placed as a one-time purchase stays a one-time order. Instead, create the subscription yourself:

1. Open Subscriptions and click Create subscription.
2. Pick the customer, products, schedule and price.
3. Set the next order date, so the customer isn't charged twice for the order they already paid.

See How to Create a Subscription for a Customer Manually.