Please make sure you have downloaded and install the Spotify app and AudFree Spotify Downloader Mac on your computer well. And then you can follow the detailed tutorial to download and use Spotify songs on iMovie easily.
iMovie supports MP3, WAV, M4A, AIFF, and AAC. You can enter to the top menu to choose 'Preferences' -> 'Convert' option, setting window will pop up. In the window, you can choose one iMovie supported audio format as output format. Besides, you can also customize output parameters as you like, such as bit rate, sample rate, etc.
To start the downloading process, please press the 'Convert' button at the bottom-right corner of the main interface. AudFree software will work at up to 5X faster speed to download Spotify songs to target iMovie files. After that, you can add music to iMovie from Spotify without limits.