Using Slim\Container::register()

script.php

<?php

// Assumes $yourExistingPimpleContainer is an instance of Pimple\Container

$provider = new YourContainerProvider($yourExistingPimpleContainer);
$container = new \Slim\Container();
$container->register($provider);

// The Slim Container should now contain all the services from your existing container

YourContainerProvider.php

use Pimple\Container;
use Pimple\ServiceProviderInterface;

final class YourContainerProvider implements ServiceProviderInterface
{
    public function __construct(Container $yourExistingPimpleContainer)
    {
        $this->container = $yourExistingPimpleContainer;
    }

    public function register(Container $pimple)
    {
        foreach ($this->container->keys() as $key) {
            $pimple[$key] = $this->container->raw($key);
        }
    }
}