Items need to be added to your upcoming subscription order before Subscription Day - the day we process subscription orders. A few days before we process subscriptions, we send out a confirmation email that reminds you about your upcoming subscription. This is a good time to add additional items to that order. Once your subscription order processes we can no longer add items to it.
You can add items to your upcoming subscription order by pressing the "Add to my Upcoming Subscription" button on a product page.