For one computer, you can use Apple’s Logic Pro X: Download additional content knowledgebase article. If you have a lot of computers to deploy the additional content to, though, you’ll want to capture those .pkg files.
One way to do that is to “install” on one machine and track down the .pkg files before doing the actual install. More details at Download all of the GarageBand / Logic Pro X Content Loops.
Another way is to use Hannes Juutilainen’s download-logicprox-content.py script to download all the .pkg files.
One you have those hundreds of .pkg files, you can import them into whatever you’re using to manage your Macs. For Munki I use this bulk-import script to get them all into the repo.
Your users may have to reindex the loops to get them all to appear.