Yes- u should be able to transfer them over then delete them off your internal. Keep the project files on the internal. When u launch the iMovie project, it should say "missing clip..." but it should give you a chance to locate it. That's where you direct it to the newly located file on your external. (I imagine it does this as it does in fcp, logic, after effects, etc etc- all work that way). If you can't reconnect the clips, you can always move them back to their original place.
The following is a suggestion. Delete at own risk:
ALSO- I have cleared TONS of space in the past by going into the Macintosh HD > Library > Application Support >(the following folders

GarageBand: Never use it? You sure? you can clear 2 gigs worth of stuff here.
iDVD: I personally don't like templates so- I toss away 2 gigs worth of huge template media here.
Now go to
Macintosh HD > Library > Audio
2.5 gigs of stuff here, but that's lots of great sfx I find myself using a lot. Your choice. Maybe could store on external? I don't know
When looking in those places- check for dupes. Delete with caution.
There's also a folder in the Library for pro apps if you have pro apps installed. DVD studio pro comes with some pretty worthless templates.
iTunes - I'm reminded of the scene in Spaceballs when the princess is told "Take only what you need to survive" and she has them lug the huge hairdryer.

Keep your favs on the internal, the rest on external.
Anyways, moving those big movie clips is a good idea. I'd stay away from deleting template stuff until you're positive you'll never need it. Good luck!