As far as I know we don't have a list with only usernames in DA.
We do have /etc/virtual/domainowners, but that is a combination of domains with owners next to it.
Best way is to use a script. Maybe you can use this or part of this:
Dear friends i need all domains and user list of directadmin on my server what to do .... can any body help me how can i do it by api if any body have scrip please provide me Thanks