While you can rename existing locations in iMovie '09 you can't add new locations. You can edit a text file hidden within the application yourself, or you can use iMovieLocationEditor to do it for you. Enter the coordinates by hand or just press the big button to copy them in from Google Earth. It's donationware. If you use it and like it please read the About box for what to do next ;-)
LicenseFree
File Size80.75 kB
Version1.03
Operating System Macintosh Mac OS X 10.5 Intel Mac OS X 10.5 Mac OS X 10.5 PPC
System RequirementsiMovie '09