No, I am talking about the private key file stored by FileZilla in unprotected format *.ppk@ModyDev by "unprotected file format", are you talking about transport encryption? Because FileZilla supports SFTP over SSH.
According to https://wiki.filezilla-project.org/Howto#Other_platforms, FileZilla works fine with standard SSH agent. I tested it on Ubuntu with XFCE+GNOME-keyring and verified that I didn't need to do any key file format conversion. The PuTTY format is only necessary on Windows because FileZilla uses PuTTY library on Windows; but you mentioned you were looking for a program for Linux.No, I am talking about the private key file stored by FileZilla in unprotected format *.ppk