Unfortunately, at the moment there is no way to access the "appDelegate.h/appDelegate.m" files out-of-the-box in AppBuilder. If you need to implement a specific native functionality using those files, your best option at the moment would be to develop a custom plugin, integrated inside your AppBuilder application that accesses the files for you.
However, you can visit our Ideas and Feedback portal and add this as a feature request as we also agree that is could be a great addition to AppBuilder. This is why we will gladly consider its implementation further once it's logged in the portal. Additionally having it there will allow the community to cast their votes thus controlling its priority.
Share what you think about AppBuilder and Telerik Platform with us in our feedback portal, so we can become even better!