I don't believe it can be done directly as MMB is bound to search already. But, depending on which mouse you're using, there might be a workaround. For example, Logitech Setpoint allows you to bind mouse buttons to keystrokes in specific applications. That means that you could set MMB to correspond to keystroke 'Ctrl+Shift+0', but only in the application Musicbee.exe. Then you use 'Ctrl+Shift+0' as a shortcut to queue next track in the playlist.