Yes! You can add one extra site to an existing subscription at a time. E.g. you have a 1-site Yoast SEO Premium subscription, but you have two websites so you'd want to upgrade to a 2-site subscription. You can do so by logging into your MyYoast account, going to Account → Subscriptions, and clicking on the green "Manage" button. On that page, you will see a link "Add another site to this subscription". This will bring you to the Yoast.com checkout page.