How do I use a custom-custom song?

Analogy:

In The Impossible Game, its level editor allows you to choose your own song by simply pressing Ctrl + A or clicking the "Open Audio" button.

In osu!, you can simply drag any .mp4 file into the editor and you can start editing a beatmap (or set of beatmaps) for it.

Question:

Is it possible to do that in geometry-dash? (Store link)

The closest thing I've found to that is this:

Which basically restricts me to anything on newgrounds.com.

Can I use a custom-custom song (one not from Newgrounds)? If so, how?
Basically, any work-arounds?
Obviously on a map that I definitely plan on not uploading, but for my personal use.

Solution 1:

1: Download any song in-game. Remember the song ID. It should be displayed in the "Saved" folder.

2: Get your .mp3 file.

3: If you are playing on a Windows computer, go to "C:\Users[username]\AppData\Local\GeometryDash" and replace the music file with the song ID you remember with your .mp3 file.

4: If you are playing on a Mac computer, go to "~/Library/Caches/" and replace the music file with the song ID you remember with your .mp3 file.

5. Use that song in-game that you changed.

6. Have fun!

(Note: This is not universal and it will only affect your song. No one else will hear the same .mp3 file as you do. Other than what I've described, there is no other way to get a custom-custom song)