Sprout Forms: Modify Entry
So it turns out you can modify an existing property like this: 

$event-&gt;params['entry']-&gt;getContent()-&gt;existingProperty = 'new value';


What I ended up doing was using a hidden field and then updating the value of that from the sproutForms.onBeforeSaveEntry event:

craft()-&gt;on('sproutForms.onBeforeSaveEntry', function(Event $event) {
    $entry = $event-&gt;params['entry'];
    $entry-&gt;getContent()-&gt;subscribeStatus = 'success';
});


Since that property can be set depending on what the response is from the cURL post, it's possible to deliver success/error messages to users on the thank you page and prevent the subscribe from silently failing in the background.