Go to Reseller Panel -> Manage User Backups and make a backup of the user who owns the domain you want to transfer.
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.
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