Adding platform to the Xamarin project already created

Yes. You will add a new project of type iOS App to your solution, then install the Xamarin.Forms from NuGet (add the version corresponding to the one you are already using in other projects).

The next step would be to replace the override content of the FinishedLaunching method with the AppDelegate class that is automatically generated so that it starts the Xamarin.Forms page. Your code will look something like this:

public override bool FinishedLaunching(UIApplication app, NSDictionary options)
{
    // Inicializa o mecanismo do Xamarin Forms
    global::Xamarin.Forms.Forms.Init();
    // Começa a aplicação iOS nativa seguindo as especificações da classe App lá do seu projeto compartilhado
    LoadApplication(new App());

    return base.FinishedLaunching(app, options);
}

If you are making use of other packages or plugins that need to boot, it is this method that you will do, but you will certainly find more information on how to do this in the package / plugin documentation.

I hope I have helped.