Is it possible to customize the mobile app settings? I want to implement a feature similar to Email app where you can add an account from app settings.
This is possible, but to my knowledge has not been done yet with a Mendix app. This is not a trivial thing to build and (in my opinion) has limited value over including preferences right in your application's menu. Therefore I would not recommend pursuing it.
However, if you still want to use a native settings menu, you would need to do the following:
add files describing the different preferences available to your iOS build, with the options defined as they should appear in the settings app, as discussed here and here