While you can’t currently add a one-off item to an upcoming subscription order, it's super easy to permanently add a new product. Here's how:
Just find the item online, select "subscribe," and add it to your basket. You’ll need to check out and pay for the order first, then from the second order onwards, the item will be discounted to the subscription price.
Make sure you log into your account and visit the "subscribe & save" section where change the dates of your upcoming subscriptions and make sure everything arrives together.