short username without domain part

==== Required information ====
- iRedMail version: 0.8.4
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): LDAP
- Linux/BSD distribution name and version:  Ubuntu 12.04
====

I have many many users and old mail server. I want to setup a new server without changes on workplaces.

But iRedmail make  username with domain part @yyy.org
How can I create box with short username ?