modules/Premium/Import is the module responsible for importing contacts/companies.
You can do your own simple script.
define('CID', false);
define('READ_ONLY_SESSION', true);
require_once('../../../include.php'); // path to include file
ModuleManager::load_modules();
Base_AclCommon::set_user(Base_UserCommon::get_user_id('user_login'));
// import file
$data = array(); // imported data
// add contact
Utils_RecordBrowserCommon::new_record('contact', $data);
You can pass a file as argument.
Remember to put this file in the place that is not reachable for the internet user.
Regards,
Adam