You can add contacts with a few simple commands, but if you wanted to use your existing list of users instead of adding them into Interspire Email Marketer that can't be done - you would have to import them in a batch using the API or Interspire Email Marketer, or add them one-by-one with the API or from within Interspire Email Marketer.
Using the API is simple. For example:
require('/path/to/iem/admin/functions/api/subscribers.php');
$subscriber_api = &new Subscribers_API();
$subscriber_api->AddToList('email@address.com', 'My Contact List');
