Nothing so generic as “User Control” (from the sound of it) exists in Cocoa Touch, except that you can directly subclass UIView to make a wholly-new kind of view.
However, you'll probably find it easier to subclass the existing UIButton, UISlider, etc. classes instead. Such subclasses should be lightweight: You shouldn't need to do more than change the control's part images (e.g., the slider's thumb and tracks).
Besides being easier, subclassing the existing classes will also mean your controls will remain consistent in behavior with standard controls. Imagine if you'd implemented your own text field from the ground up prior to Apple adding copy and paste.
Once you've made your subclasses, you should be able to select your controls in IB and set their custom class names to the names of your custom subclasses. You should then be able to add those controls to the Library to reuse them. (Caveat: I only write Mac software, so I don't know whether IB has these features for iPhone development.)