Any customers that are in MYOB who aren't in Magento will be added and their CardID updated to match.
You can try a fully function trial of the Export Customers free for 30 days. You can purchase the full version of Export Customers for NZD $49.00+GST.
All Unicart Extensions are not stand-alone and require a licenced version or current trial of Unicart.
Unicart Extensions are version specific. After you update to the latest version of Unicart you should always update to the latest version of the extension.