I'll be curious to see if someone replies with an easy method to do this. Because as you know, iTunes puts the converted files in the main Library.
You could search thru Doug's Applescripts for a solution
Or with an extra step on your part you could create a Smart Playlist. For the songs you want to convert, Get Info and add a comment like "convert to mp3. Then create a Smart Playlist to look something like this. Of course this is not as simple a solution as you're looking for, but might be a temporary solution.