Composer : how to add a dependency without network connection?

Thanks to edmondscommerce's comment I found the solution:

I update my main composer.json file with an artifact respository (and I disable the packagist one):

{
    "name": "user/silex",
    "repositories": [
        {
            "type": "artifact",
            "url": "artifact/"
        }, {
            "packagist": false
        }
    ], "require": {
        "silex/silex": "1.2"
                , "twig/twig": ">=1.8,<2.0-dev"
                , "monolog/monolog": "1.*"
                , "doctrine/dbal": "2.2.*"
                , "symfony/security": "~2.3"
    },
    "autoload": {
        "psr-4": {
            "Portal\\": "src/"
        }
    }
}

Then I put a folder called artifact according to the url put in the composer.json file.

I create into this folder a zip called monolog-monolog-1.8.zip with the library I want to add.

Then just launch a composer update command!

Be carefull, zip's root must contain a composer.json file, and this composer.json file must contain a version!