Log into your account.
After successfully logging in, this will bring you to your dashboard homepage.
Click, "My Pet and their subscription", then in the top right corner click the yellow "Add a pet", button.
Follow the questionnaire for "Add a pet". Upon completion, your new pet will be added to your subscription.
Adding a new pet will not generate a new order for them, so they will be added to your next order with Itch. You can check this under the 'my orders' tab.