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.
/** Your lumen composer.json **/
"autoload": {
"psr-4": {
"App\\": "app/", //Lumen namespace
"Laravel\\":"../Laravel5/app" // Laravel namespace
},
/** Run composer dump-autoload -o **/
// FROM YOUR LUMEN APP YOU CAN USER ANY CLASS UNDER Laravel namespace
// Then in your Lumen/app/Http/routes.php you can call laravel classes like below
$app->get('api/v1',function()
{
return \Laravel\User::all(); //Note that I Started namespace with \
});