Thanks for your suggestion but I do not think there is a way to import music directly from YouTube into any app :)I am not sure if it would even be legal to do that :)
Yeah not legal.
You could possibly do 3 things however:
1) Download the song from youtube via some online converter to your computer, send it over to your device & add it in.
2) Download the song from youtube via some online converter to your computer & add it to the final animation (that you sent from your device to your computer) when you compose it with any video programme (on your Computer).
3) Better, use some free for personal/commercial music that you can actually legally use for your animation work. This way you can ensure to upload it to Youtube without having to worry about a copyright strike from Youtube which could lead to the deletion of your video. For this simply search "Royalty Free Music"