I can imagine the following will do the trick:
Go to the File menu and choose Preferences, there choose 'advanced' and in advanced choose 'importing'. There you choose from a pull-down menu 'mp3 encoder'.
Click 'OK' and then select the track you want to convert in the iTunes library. Then go to the 'Advanced' menu and choose 'convert selection to MP3'. Alternatively you can right-click (control-click) the track you want to convert and choose 'convert selection to MP3' from the contextual menu.
Hope this works!
V