What you could try to do is, if you have the mp3 file, or somehow obtain it, add it to Spotify via local files. It can only be done on PC, however once done on PC, it can be accessed on other devices.
To add songs via local files, find the mp3 file(s) you want to add. Make it easier by making sure the file(s) are in the root of your music library. Then, go to local files on the right side bar. Find your song, right click and add it to a new playlist. You'll be adding all local songs onto that playlist. Then go to said playlist and make available offline. Once you've done that, then you can add the song to whatever playlist you want.
I hope I helped, thanks for reading.
