Hi there. Are you talking about an actual package installer, like a file with a .pkg extension on it? If so, there aren't really any ways to easily add in some other files without repackaging it. But repackaging it could be as simple as pulling the .pkg into Composer and converting it to a source, adding what you need to it, and then rebuilding it as a new .pkg installer. There are also some command line ways to adjust packages, but that's probably unnecessary to go into. You can use the same name as it had before for the package name.
if OTOH, you meant how to add something into a policy that installs this package, adding extra packages to those is easy. Just go into the policy, edit it and under the Packages section on the left, click the + button to select another package to add. You will have to make sure the package is added to your repository first of course. You can have more than one package in a policy.
In the latter case, the policy name would remain the same, and end users, assuming they are installing this from Self Service, would not know anything has been changed or added to it. It wasn't clear if that's what you meant by not wanting to retrain your user base or not.
Thanks MM2270. That is excellent information. The second scenario you mention - adding something to the policy - should do the trick. I appreciate the information.