Då tror jag du ska göra så här:
"iCal saves your calendars as .ics files in the Calendars folder (~/Library/Calendars). You can back up your calendars by copying this folder to your backup location (file server, external hard drive, CD-ROM, and so forth)."
Dvs flytta denna mappen till nya installationen så ska kalendrarna ploppa upp i iCal.