To bulk upload subscriptions to your customer record, Go to
> Settings
> Import and Export Data
> Choose a bulk operation
> Import subscriptions for Items (if PC 2.0)/ Import subscriptions for customers (if PC 1.0) and click on Proceed. 
> Download the sample CSV
> Enter the details 
> Upload the CSV 
You can find the detailed steps in this document. Once the action is done, all the customers mentioned in the CSV will have the subscriptions attached to their records.
 

For Pc 1.0

For PC 2.0