If you would like to add emails to one of your Opt-Out Lists in real-time via an API, you can do this through our standard Opt-Out collection process. You also get the benefit of Opt-Out analytics and graphs when using this process.
First, you simply create a Mailer and Campaign (e.g. named 'Central OptOut System') and add that Mailer to the Campaign, as you normally would, for each Opt-Out List. Then log in to the Mailer's interface for that Campaign and download the list to grab an Opt-Out Link.
Then you can use the one-click-unsubscribe feature to append the email address to the end of the Opt-Out Link. To turn this into an API call, simply add an api=1 param at the end, which will output xml instead of the html confirmation page (example below).