Olá Marcus,
Vai na segunda página que você encontra o prefixo_user 
mdl_user (padrão) - > instalação padrão.
Ai da um Big insert como INSERT INTO `mdl_user`(`id`, `auth`, `confirmed`, `policyagreed`, `deleted`, `suspended`, `mnethostid`, `username`, `password`, `idnumber`, `firstname`, `lastname`, `email`, `emailstop`, `icq`, `skype`, `yahoo`, `aim`, `msn`, `phone1`, `phone2`, `institution`, `department`, `address`, `city`, `country`, `lang`, `calendartype`, `theme`, `timezone`, `firstaccess`, `lastaccess`, `lastlogin`, `currentlogin`, `lastip`, `secret`, `picture`, `url`, `description`, `descriptionformat`, `mailformat`, `maildigest`, `maildisplay`, `autosubscribe`, `trackforums`, `timecreated`, `timemodified`, `trustbitmask`, `imagealt`, `lastnamephonetic`, `firstnamephonetic`, `middlename`, `alternatename`) VALUES ([value-1],[value-2],[value-3],[value-4],[value-5],[value-6],[value-7],[value-8],[value-9],[value-10],[value-11],[value-12],[value-13],[value-14],[value-15],[value-16],[value-17],[value-18],[value-19],[value-20],[value-21],[value-22],[value-23],[value-24],[value-25],[value-26],[value-27],[value-28],[value-29],[value-30],[value-31],[value-32],[value-33],[value-34],[value-35],[value-36],[value-37],[value-38],[value-39],[value-40],[value-41],[value-42],[value-43],[value-44],[value-45],[value-46],[value-47],[value-48],[value-49],[value-50],[value-51],[value-52],[value-53])
O colega Lino tem uma boa documentação sobre SQL e Moodle em http://moodlesql.blogspot.com.br/2010/07/cadastrar-usuario-no-moodle-pelo.html
Sucesso!
