Recently I've figured out a method for using ingame custom music to synchronize coasters to a song.
The video outlines how to do it, but if you don't have the time, here's a quick rundown. (However, I'd recommend watching the video for a more visual guide)
1. Get the song you want to use ingame.
2. Put the song in an audio editor and add a few seconds of quiet before the song starts. This makes it easier to time the start of your coaster to the start of the song.
3. Render the song out as a .wav, with the following set