It sounds to me that you may not actually want to use a UINavigationBar. As it states in the reference documentation:
The UINavigationBar class implements a control for navigating hierarchical content. It’s a bar, typically displayed at the top of the screen, containing buttons for navigating up and down a hierarchy. The primary properties are a left (back) button, a center title, and an optional right button. You can specify custom views for each of these.
So if you're planned customizations go beyond adding buttons, changing it's color / background, opacity, hiding etc.. you might be better off creating a UIView that mimics the look & feel of a navigation bar.
Here's an example of how to give your UIView that gradient look of a navigation bar.
It's far more flexible and actually quite easy to do BUT you've got alot of reading and testing ahead of you :).
Just in case if it's just buttons you're thinking of adding you might be better off using UIToolbar instead