One way to reuse clips among iMovie projects is to Option-drag their clip files among the projects’ Media folders, thereby making copies. But if you’d rather not switch to the Finder and muck around in folder windows, you can use the File → Import File command to simplify the process.
When you choose it, the Import File dialog box appears. Navigate to the folder that contains the project whose footage you’d like to steal, and open it. Then open the Media folder inside, and double-click the Clip file you want to import into the project that’s open onscreen. After a moment—a long moment, as iMovie copies this enormous file into the current project’s own Media folder—the imported clip appears on the Clips pane.
Of course, these cheerful instructions overlook one massive roadblock: figuring out which of the dozens of clips in the raided project’s Media folder is the one you want to import. After all, they don’t have descriptive names and you can’t preview them.
That’s why the importing-a-clip-from-another-project routine generally involves a detour: opening the other project, finding the clip (in the Clips pane or Movie Track) that you’re trying to reuse, double-clicking it, and then reading its clip number in the Clip info dialog box. Then you can return to your newer project and import the clip as directed above.