Thanks for using Apple Support Communities.
For more information on this, take a look at:
iTunes: About the Add to Library, Import, and Convert functions
With iTunes, you can convert music files from one file format to another. You can also convert a file to the same format using different options. For example, you can convert high-bitrate MP3 files to a lower bitrate to fit more files on an MP3 CD.
Note: You can't convert protected music that you purchased from the iTunes Store.
Best of luck,