How to use a Mac OS X Server as a cache for Software Updates
Use these steps to point client Macs that aren't managed with a configuration profile or Managed Client to your Software Update server:
On the unmanaged client, open Terminal.
Use the following command. Substitute your Software Update Server's fully qualified host name for su.example.com.
sudo defaults write /Library/Preferences/com.apple.SoftwareUpdate CatalogURL http://su.example.com:8088/index.sucatalog
You can verify your change using this command:
defaults read /Library/Preferences/com.apple.SoftwareUpdate CatalogURL
To point an unmanaged client computer back to the Apple Software Update server, use this command:
sudo defaults delete /Library/Preferences/com.apple.SoftwareUpdate CatalogURL