If you want to export an audio file from iMovie to Garageband, follow the below steps:
•Create an audio file in iMovie using the aforementioned method and then, go to Share Menu and select Export Movie.
•Choose the size of the export file, most users select the Medium option.
•Now, choose Garageband in the “Where” menu and hit Export.
•Next, launch Garageband and confirm that the Movies is selected in the top-right Media Browser section.
•Now, drag the file to the top track from the bottom right box.