In order to transfer your Garageband  soundtracks to iMovie , first you will have to convert the file to a .mp3 so that it can be opened in iTunes .

1. Open the Garageband  project you wish to use in your iMovie  and click Share > Song to iTunes... .
   
    
2. Name your document and Choose the Location where you wish to Save your document. Click Share.
   
    
3. Open iTunes  and the file you just created will appear under My Music.
   
    
4. Now, Open iMovie  and select Import Media.
   
    
5. Find the iTunes  .mp3 document you just created and select Import All.
   
    
6. Now Create a New Project (or find the current project on which you are working).
   
    
7. Select the theme (if you're making a new project) and click Create.
   
    
8. Create a title and Select OK.
   
    
9. In the top left hand corner Select the sound clip you wish to Add to the iMovie .
   
    
10. Click and Drag this down to the bottom editing panel.
    
     
11. The sound file has now been Added to your movie. You can edit it from this bottom panel if you need.