Updating Symantec Enpoint Protection's Proactive Threat Protection Without an Internet Connection
From http://www.symantec.com/connect/forums/sep-avptpntp-definition-files-client-machines:
It looks like you can copy this directory (or its Windows Vista/7 equivalent) from an updated machine to your other machine.
C:\Documents and Settings\All Users\Application Data\Symantec\SyKnApps
I'm not sure if you have to shut down the SEP during the process, and don't have it available to test, but that looks like a reasonable solution.