Sometimes you need to install an Eclipse plugin without an access to the Internet or -perhaps more frequently- with an (extremly stupid) firewall that blocks the traffic and prevents the download (and additionally destroys your productivity as developer by pretending all the technical subjects are related to hacking :p)
Unfortunately, the website for this plugin doesn’t offer a zip file in order to install it offline. And your network administrator -who doesn’t want to understand anything in development- has all the power (you’re completly out of luck, maybe you should consider another company rather than be reading this :-)).
So, here is what you can do in order to create a mirror of the update website, from another computer that has Eclipse installed and that has access to the net:
/eclipse -application org.eclipse.update.core.standaloneUpdate -command mirror -from http://www.aniceplugin.com/update-site -to /path/to/local/directory
The various commands for the update manager are further detailed in the Eclipse documentation under the section "Running update manager from command line".