First, go to iTunes Preferences > Advanced > Importing and change the import encoder to MP3 (and whatever sampling rate you wish). Important: everything that you wish to import will be in MP3 until you switch this back to AAC.
2nd, select the tracks you wish to convert; go to the Advanced menu > Convert Selection to MP3
It will convert a copy to MP3 - not remove the existing AAC file - so you'll have two songs.
If you sort by date added, you'll find the MP3 files as being most recent.