If you use UIBarButtonSystemItemAdd, you should be able to tap a fairly large distance away from the button. Here's a screenshot showing where I was tapping, and as you can see the button is pressed:
A button like this can be created using code like this:
[[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemAdd target:self action:@selector(addPressed:)]
You can also create one in interface builder: