Yes you can.
1. In iMovie consolidate projects and events (http://help.apple.com/imovie/mac/10.0.8/#/mov882dee351) to make sure everything is copied to the library
2. Quit iMovie
3. Copy the iMovie library to your external HD using the Finder
4. Send the old library (in <user>/Movies) to the trash but don't empty it
4. Launch iMovie Open Library > Other and navigate to the library on the external HD.
6 Check that everything is working
7. Empty trash
You should of course anyway have a library backup on a separate storage device
See: http://help.apple.com/imovie/mac/10.0.8/#/mov3fa25bae7 for more details on managing libraries
Geoff.