If you want to add Lumen to your laravel 5 and you don't want to create new repositories models controllers etc... Just create a new Lumen project and autoload laravel namespace in lumen composer.json I assume both laravel5 and lumen are in the same directory but each one in it's own project directory.

Let's say, that your user provide inputs which has arrays like multiple accounts and different crediting and debiting amount, and you need to validate if they are balancing. You will need to extract the array and do some sort of manipulations before proceeding to validation...You can just overload the all() method in your form request class