Itunes is a easy way to convert files to lossy formats. I would first burn your files to CDR as wav and then convert to mp3.
drop files in music or use cd
preferences>
Advanced>
Import>
check Import using (mp3,AIF,wav,etc)
check Setting (quality of mp3 if using lossy)
Okay
select files to covert
click advanced>convert selection to (your choice of import)