If you want to import just a few subscriptions, you can do this by placing an order on each member's behalf for their subscriptions. If it's a past subscription, and you don't want to record the financial transaction, then in admin checkout, add an adjustment to the order to reduce the price to £0. You may want to untick the notify box, so that the system doesn't send the member an email. If the order has a financial value, you will need to update it to Payment Received for the subscription to be created.
Note: the subscription will be created with an end date in the future (determined by what you configured for the subscription product). If in fact you are creating last year's/month's subscription, then you can edit the subscription to change its end date.
If you want to import a lot of subscriptions, then contact us about a bulk subscription import.