To add an item to your existing subscription, you should add the items you wish to subscribe to in your cart.
At the checkout page, select the option to receive these items via subscription.
Scroll down past the cart total, and you should see the option to Add this cart to an existing subscription.
A list of your existing subscriptions will appear. Click to Add your cart to the subscription of your choice and it will combine your subscription.
You should see this reflected at the next page, where you are directed back to the My Subscription tab in your account.