You can convert monthly subscriptions to a pre-paid subscription by following this guide. You will need to adjust the charge as well as the shipping frequency, then add in the $0.00 orders. This is not a solution if someone needs to be added to ReCharge partway through their pre-paid cycle.
In the ReCharge Dashboard under Customers, search and select your customer, then click See Details beside the Address ID.
Click Edit Recurring Properties to change the interval.
Click Pre-Paid Frequency Settings and configure, remember to Save.
Note: The charge interval should be larger than the shipping interval.
On the Purchase page for this customer, click the latest charge and on Add Shipment to add the remaining $0.00 orders scheduled for the current cycle.
Adjust the charge date of future orders if needed by clicking the order and on Change Date.
Here is an example of a 6-month pre-paid subscription: