adduser problem

["Tony Leung" <vsctony@hotmail.com>]

Dear All,

I want to create user account with dot between the username, but can't create, what setting should i change? 

test:~# adduser abc.ken
adduser: Please enter a username consisting of a lower case letter
followed by lower case letters and numbers.  Use the `--force-badname'
option to allow underscores, and uppercase.

Thanks,

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