The easiest way to add an item to your subscription is to log into your account and manage your subscriptions. There is an Add Product button on the subscriptions page of your account. Search for the desired product, select the frequency, and start the new subscription.
Another option is to place a new order and select the Subscribe & Save option when adding to your cart. After completing your purchase, you can visit your account page to adjust the date and frequency of your subscription.
Just a note: Customers often find it most convenient to set up their subscription items to process on the same date so they ship together.