As far as I know, .sav files are encrypted and require the use of the rom to decrypt it. For example if this save came from Gateway, you'd need a Gateway plus the rom of that game before you can decrypt it and export it for use with save managers.
You should ask someone to convert it for you. I did the same thing and I got help really quicly.
Tu devrais demander de l'aide à quelqu'un sur le site. J'ai fait de même hier et on m'a converti ma sauvegarde très vite. (Juste au cas-où tu préfères le français x) )
P.s. Before anyone starts saying that I shouldn't speak other languages here, I only translated what I said in the first sentence because I assumed he's french(because of the displayed flag and his profile picture).