I don't know of a quick way, but I might think about it like this.
1) Create a Smart Album in iPhoto that contains only movies.
2) Right-click on each one and choose Reveal in Finder. Make a copy and paste it in a folder where you an find it.
3) Import these movies into iMovie using FILE/IMPORT MOVIE
4) Go back and delete the movies from ipHoto. You can also delete that Finder folder you made as an intermediate step.