You can't add anything to those packages. You can only develop stuff on packages for which you have the source code. So, you need to create your own package and model if you don't already have a custom model that you are working on.
Where did you get the deployable package? Is it a third party solution? Ir something your company developed? If it's your company's code, you need to get the source code from source control. That's how you share code between developers.
All your custom code and all ISV solutions (binary or model) should anyway be added to your source control. Then your build process should build one package from all that stuff. Then you deploy this package to test/prod.