Changing following setting in the file: catalog/admin/includes/languages/greek.php
define('CHARSET', 'iso-8859-1');
to
define('CHARSET', 'iso-8859-7');
enables you to add Greek characters without a problem in your admin
You also need to make changes in
@setlocale(LC_TIME, 'de_DE.ISO_8859-1'); ..this is for German language..you need to make change for Greek language