Presumably you have been using iMovie 9.
First you should consolidate your media from within iMovie 9 on the hard drive it is on now so that the iMovie events and projects folders have copies of everything you need and no links which will break when you move it to another disk. See:
You can then copy the folders to the root level of your external hard drive.
When you connect this hard drive to your mac Pro (I'm jealous!), imovie 10 should 'see' the folders and offer to update your projects and events to an imovie 10 library. See:
If you have a lot of projects and media it may be best not to update everything at once but in stages. Each update session will go into a different library. I can explain further if necessary.