There is a new script in testing for this. Sample usage:
cd /usr/local/directadmin/scripts ./move_domain.sh domain.com olduser newuser
Note, that if you have any apache owned files, they'll be chowned to the new user. Also, if you have apache_public_html=1 in your directadmin.conf, I would recommended to change it to: apache_public_html=0 and use the secure access group method instead.
older outdated method:
1) Go to Reseller Panel → Manage User Backups and make a backup of the user who owns the domain you want to transfer.
2) Login as that user, go to User Panel → Domain Administration → Change domain name, and change the name of that domain to something different (ie: from domain.com to domain2.com). This is done so that you can restore the domain on the other account without DA complaining that it already exists.
3) Go back into the Reseller Panel, and create the a new user. Set the domain to the actual name of the domain you want transferred. If the user already exists, then just create the domain through the User Panel → Domain Administration menu.
4) Take the backup from step 1, and rename is from olduser.tar.gz to newuser.tar.gz, where newuser is the name you are transferring TO and olduser is the name you are transferring FROM.
5) From Reseller Panel → Manage User Backups, select newuser.tar.gz and restore it into the newuser account. This should set all the data from the domain into the new user.
6) Once you've verified that everything is correct and as it should be, log back into the olduser account and delete domain2.com.