There is a way to share users and therefore share the domains under that user. But when doing this one user has control over the other user's entire account.
For instance as a test I created a reseller. Then edited /usr/local/directadmin/data/users/testresell/users.list and put in admin. I then logged in as testresell and from there I could login as admin. But that's not good because then he has access to the entire admin account.
I think from the example above you get the idea of what is possible though. But you don't want to "share" the admin account. You need to move the domain to another user first.
I just want to add that it's possible to set up a separate ftp account to be able to ftp to public_html, without that user having any access to the admin (or any other) entire account.