customize live cd and add custom user name with default password

For some professional reason, we need to make our simplified live ubuntu version. (It has to be live, can’t be installed on flash drive)

In this link they explain how to disable the auto login, https://help.ubuntu.com/community/LiveCDCustomization#Removing_the_.28Casper.29_Autologin but now I need to create a default username and password, so how can I achieve this?

Thanks in advance