ComAge, what you describe is not overselling.
Overselling is when you set a user for 10GB and let them have 20GB.
What you're asking about is can you give a user 10GB and yet have the space not dedicated to them until they need it.
That's actually a function of the base OS, either Linux or FreeBSD.
And yes, that's the way DirectAdmin works, because that's the way the underlying OS versions work.
And this allows you to oversell if you'd like.
Note that if you want to oversell you should NOT have quotas enabled on your server.
Jeff