You'll need to write some code for that. Create a copy of the UIDragObject script, or edit the existing one.
Personally I wouldn't use it for an on-screen joystick. I'd write a custom script for it, as a joystick also needs to spring back to the center when released.
Something like this would be trivial to do -- custom script with an OnDrag function that moves some target sprite, keeping it within X distance of the desired center. When released, snap or SpringLerp back to center.