For this you could hook in to the LoginEvent.
Run:
php artisan make:listener AddToSessionAfterLogin
Then n your App\Providers\EventServiceProvider add the following to the $listen array:
\Illuminate\Auth\Events\Login::class => [
\App\Listeners\AddToSessionAfterLogin::class,
],
Obviously, feel free to import these classes if you want.
Then in your AddToSessionAfterLogin class, which you've just created, you can add the necessary logic to the handle() method i.e.:
public function handle(Login $event)
{
session(['hello' => 'world']);
}
FYI, you can access the authenticated user with $event->user.