The system allows you to import and export a set of codes in CSV format.
Going to the "Manage Codes" option on the Admin interface, by the end of the page there is a "View All Codes" button, when clicking the button, you will see as screen like this: Just click the "Download CSV file" and you will get the CSV file with all the codes you have generated.
To upload a list of Access Codes, you need to go to the "Manage Codes" option on the Admin interface, just before the Check/Delete Codes box by the end of the page, there is a link "Import Codes". By clicking the link you will be redirected to this page: