If you have an Akismet Plus subscription, you can add sites to it at any time by activating Akismet using your API key. Because Plus subscriptions are capped per monthly API calls, you may want to make sure the total API calls made by your sites don』t exceed your tier.
If your API calls are exceeding your tier, you can upgrade to have a higher one. Once you log in to your account and locate your Subscriptions section, click on the Edit or Upgrade link to modify your Plus subscription.
Use the Subscription Level drop-down menu to select the number of monthly API calls that you need to cover with this subscription, and click on the blue Submit button.
You can use this to update your card on file and change the billing from monthly to yearly and vice versa.
If you have trouble adding sites to your account or if you have any other questions please contact support.