I found it via google, could not find a good search term for searching this forums for you.
I tried "create accounts without domain" with 10 pages or so result.
create user without domain site:www.directadmin.com
However I can't find a thread which answers the question so I'll answer it here.
Simply create (perhaps using a custom skin and and API calls) a user with a bogus domain (for example USERNAME.example.com (where USERNAME is replaced with the specific username being created) and then call the API call to delete the domain immediately.
For a user I don't see much in the way of issues, but if you do this with a reseller you won't be able to send a new account email, ticket notification, or system message, without a meaningful email address unless you set one up manually.