Learn how to move clips from one iMovie project to another.
In iMovie HD 6, you can open more than one project at a time. Simply open both projects, and drag the items from one iMovie project window to the other.
Tip: Only drag from one track at a time and drag any audio tracks separately.
Alternatively, you can select the item and cut (Command-X) or copy (Command-C) it, then paste (Command-V) it into the other project window. Be aware that copying and pasting audio could result in overlapping audio (or even a longer piece of audio obscuring or covering up a shorter piece of audio).
In iMovie HD 5 you can copy and paste items between projects. Select the item you want to move, and cut (Command-X) or copy (Command-C) the item. Close the originating project, and open the project where you want to place this item. Paste the item (Command-V) to import the clip.
In iMovie, you can export or share the desired clip as Full Quality. Then, import the resulting file into the other project. Tip: iMovie 4 and later will allow you to share just selected clips. Consider selecting just the clip you wish to move to the other project, and use this option.
Alternatively, locate the resulting file in the project's Media folder in the Finder, and import this file into the other project (in iMovie, File > Import).