How to make a song use offline

Hey Jecollins37,
I understand that you have a song that is not downloaded to your iPad and would like to add it to your iMovie Project. Let's go over how to get songs downloaded your iPad.
If the songs were purchased from the iTunes Store, you can download them right from the iTunes Store app. Open up that app, tap Purchased, Tap Not on This iPad and then locate the Music that you want to download. Tap the Download icon to get it on your iPad. Once it is there, you can use it with your iMovie.
Download your past purchases
https://support.apple.com/en-us/HT201272
I have also provided the steps to add it to iMovie on your iPad as well in case you need that.
Add audio
http://help.apple.com/imovie/ipad/2.2/#/kna257fc2a9​

Add background music stored on your device

You can add any of the following types of background music to a movie project:

• Theme: Music that comes with each Apple-designed iMovie theme

• iTunes: Songs from iTunes that have been synced to your device using iTunes on your computer

• Imported: Songs that you created in GarageBand, Logic Pro X, or another app and imported into iMovie using iCloud Drive or iTunes on your computer. (To import background music from iCloud Drive, see “Add background music or other audio from iCloud Drive,” below.)

Note: Songs that appear dimmed or are marked Unavailable need to be downloaded to your device for use in iMovie. Use the Music app to download songs from iTunes in the Cloud, or use iTunes on your computer to sync songs from your iTunes library. If a song on your device still appears unavailable, it may be of a type that can’t be used in iMovie. For example, songs protected by digital rights management can’t be used in iMovie.

Background music clips are colored green in the timeline. If you add a song that’s less than 1 minute long, it behaves the same as a sound effect in the project and is colored blue in the timeline.

1. With your project open, tap Audio in the media library.

   Note: If the media library is not visible, first tap the Media Library button in the upper-right corner, then tap Audio at the bottom of the media library.

2. To find music previously synced to your device from your computer, tap Theme Music or any one of the listed categories.

3. To listen to the music before adding it, tap the item, then tap the Play button .

4. To add the music to your movie project, tap the item, then tap the arrow .

Tip: If you’re creating a music video and want to align video clips to specific points in the music, add background music to your project first. (If the music is shorter than 1 minute, you must add video to the project before you add the music.)

By default, iMovie automatically loops background music to fit the length of your movie. You can add multiple songs as background music, but iMovie automatically arranges them to play one song at a time. You can also manually adjust the length if you want.

Take it easy