Yes there is. Find the file named user_premium.php in the folder sources/user_cp/ and find this line:
$fp = fsockopen ('ssl://www.paypal.com', 443, $errno, $errstr, 30);
then modify to:
$fp = fsockopen ('ssl://www.sandbox.paypal.com', 443, $errno, $errstr, 30);
Upload modified file to the...