Works until iOS11!
You can use negative flexible spaces and rightBarButtonItems property instead of rightBarButtonItem:
UIBarButtonItem *spacer = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFixedSpace target:nil action:nil];
spacer.width = -10; // for example shift right bar button to the right
self.navigationItem.rightBarButtonItems = @[spacer, yourBarButton];