DirectAdmin isn't exactly noted for having great documentation.
To back up an account from the command line run:
/usr/local/directadmin/directadmin taskq --run="action=backup&append_to_path=nothing&database_data_aware=yes&email_data_aware=yes&local_path=%directory%&owner=%owner%&select0=%user%&trash_aware=yes&type=admin&value=multiple&when=now&where=local"
Changing the %directory%, %owner%, and %user% variables as necessary.
To restore an account from a backup:
/usr/local/directadmin/directadmin taskq --run="action=restore&ip_choice=select&ip=%ip%&local_path=%directory%&owner=%owner%&select0=user.%owner%.%user%.tar&type=admin&value=multiple&when=now&where=local"
Changing the %ip%, %directory%, %owner%, and %user% variables as necessary.