I have not heard of this feature being implemented yet. Although I can appreciate that it would be helpful, the process (for 450GB) would probably take days!
Rather than convert to MP3 on the B2, it would be much easier to convert from your backup drive. I use dBpoweramp Music Converter, and it allows you to select the MP3 bitrate and to name the destination folder.
If you do not have a backup drive with all your B2 music on it, you're playing with fire. It's not a question of IF your B2's (or any other) HDD will fail, it's a matter of WHEN.
If you don't have a JB7 where you'd need backward compatibility, use the B2's Export command to do your backup.