Your external rive must be formatted as Mac OS Extended (journaled). If not, you can reformat using Disk Utiltity.
On the iMovie Import Screen, you can select an external drive as the target for the import. This will remain the default place for importing events until you change it.
For Events that you have already imported, you can move them to an external drive. This must be done from within iMovie, so you do not break the links to the projects.
In iMovie, click VIEW/EVENTS BY DISK. You should now see your Event Library with small icons for available external disks. Hold down the Command Key as you drag the Event icon to the icon for the external drive in the Event Library List. (If you do not hold down the Command key, it will Copy, not Move).
I also suggest you look in Help at the Consolidate Media command. This is a good way to put all Project assets into the same external drive.