Added the following code to the listSubscribe() function in CC_SuperClass. This allows me to add to my lists, rather than replacing them:
// Check if email already exists; update if it does
if($existingID = self::CC_Contact()->subscriberExists($params['email_address'])) {
//added next three lines to ensure our new selection adds to our subscribed lists, instead of replacing them...
$contactDetails = self::CC_Contact()->getSubscriberDetails($params['email_address']);
$contactLists = $contactDetails['lists'];
$params["lists"] = array_unique(array_merge($contactLists, $params["lists"]));
//end of new code
$contactXML = self::CC_Contact()->createContactXML((string)$existingID,$params);
Thanks, @mvbaxter – sorry I didn’t see your code sooner!
This is also fixed in version 2.0.1, released today.