Enable custom DNS after adding domain?

[kaotix]

I recently added 2 separate hosting packages for my site so I could let certain users that I know personally to test out the custom DNS zones within their panel.
Someone else has requested this feature but already has their site setup on the server. I cannot for the life of me find out where to enable this for a user already setup.

Short of editing the database is there somewhere within the panel I can change it?