instead of add you can use merge like this:

public function getForm(TwillModelContract $model): Form
    {
        $form = parent::getForm($model);

        return $form->merge([
            Input::make()->name('description')->label('Description')->translatable(),
            Medias::make()->name('cover')->label('Cover image'),
            BlockEditor::make()
        ]);
    }

$form is a laravel collection so you can use the same methods. merge returns a new instance so you have to return or capture&return it.