Any user you create on a Linux or BSD server should automatically have an email account setup for it.
But I presume you're looking to create an email account for a Virtual domain hosted on the server. I'm positive it can be done; there's nothing magic in DirectAdmin; it's all done with configuration files and files in /etc/virtual.
But it's a lot of work to figure out exactly what files need to be modified. Perhaps someone has it figured out and will replybut it's been a few days. Buying a license for a month is probably a lot cheaper than paying me or some other administrator to do it without an exact set of files.
But if you want to try, then look at files in and under /etc/virtual.
Jeff