I don't use iTunes, nor do I ever wish to, but here is the answer that Google gives me for your question. Hope it is still relevant. http://askbobrankin.com/convert_itunes_to_mp3.html
And here is an answer given in the discussion on that page:
There is a way to convert all your purchased iTunes AAC files to MP3 without downloading or buying any program to convert it. I will try to simplify the steps below, but I am in a hurry...
1. Open iTunes, click edit, then preferences, on general tab click on import settings and change the Import Using to MP3 encoder
2. In upper left corner there is an arrow, click on arrow and then click on "show menu", then click on the "view" tab and click on "show sidebar"
3. Under Library click on "music", then you will see a column "Name", right click on it and look for "Kind" and "Artist" and make sure you click a check mark next to them. This will now create "Kind" & "Artist" columns if you do not already have them.
4. click on the Artist column to put them in artist order, then look for the song under the artist (not the song name and you will find your song which will also show it is in AAC format.
4. highlight the song, then right click and hit "Create MP3 version" and it will create the same song to MP3 and you will now have 2 copies of that song, one in AAC format and the other in MP3 format.