You can find the location where the nbm files are stored in the Plugins window (Tools/Plugins). On the Settings tab the repositories are listed:
You are probably only interested in the SNAP and the SNAP Extensions repository.
Copy the URL and remove the updates.xml.gz part. Now past it in your browser. All nbm files will be listed. With some tool like DownThemAll, or so, you can get them easily.
You can then have those files On a USB and use the Installed tab to install these updates.
Another option is to put them on a network drive (along with the updates.xml files) and add the path to the xml to the list of repositories. The others should be removed. I haven’t tried this option yet, but I think it will work.