Making MP3's in OSX

I've got an odd request.

Nearly all of my music is ripped from CD into Apple Lossless format for inclusion in my iTunes library.

Now I will be using a car stereo that will play MP3, plus a couple others (but not Apple Lossless).

I want to make a select few MP3 tracks to dump on a USB stick for this purpose. I have the original CDs, or i can convert from the files currently in my library. However, I want to take a lot of caution to NOT mess up my current library.

What's the best way to do this? I know I could import into iTunes but then I'd almost certainly mess up my existing files. Thanks for any insight.