While you can't add new items directly to an existing subscription, you can sync your delivery dates after checkout to group future shipments.
To do this, you'll need to place a separate order on our site for the new item to start its subscription. Please note that your first order for the new item will process right away and ship separately.
Once created, you can easily align all future delivery dates so everything ships together!
How to combine future shipments
Go to your Dashboard or Subscriptions tab.
Click 'Reschedule Shipment' on the new item.
Select the same ship date as your existing subscription.
You'll see the subscriptions merged on your Subscriptions page.