I've just submitted v3.0.0 of Add Customers from Admin for the Zen Cart moderators' review; I'll post back here when it's available for download.

This version (see https://github.com/lat9/add_customers_from_admin) is a heavy rewrite/refactor and now requires Zen Cart 1.5.7 (or later) for its proper operation.

Various PHP notices, warnings and deprecations were corrected.
The processing in the admin's add_customers_backend.php has been moved to /admin/includes/classes/addCustomers.php