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