Add Zip package to drupal using composer (Example : CKEditor colorbutton as drupal-library)

Add following packages to composer.json file next to drupal repository

"repositories": [

{

    "package": {

        "name": "ckeditor/colorbutton",

        "version": "4.16.2",

        "type": "drupal-library",

        "dist": {

            "url": "https://download.ckeditor.com/colorbutton/releases/colorbutton_4.16.2.zip",

            "type": "zip"

        }

    },

    "type": "package"

},

{

    "package": {

        "name": "ckeditor/panelbutton",

        "version": "4.16.2",

        "type": "drupal-library",

        "dist": {

            "url": "https://download.ckeditor.com/panelbutton/releases/panelbutton_4.16.2.zip",

            "type": "zip"

        }

    },

    "type": "package"

},

{

    "type": "composer",

    "url": "https://packages.drupal.org/8"

}

],

Then run:

composer require ckeditor/panelbutton

composer require ckeditor/colorbutton

Also see : https://getcomposer.org/doc/05-repositories.md