It has been a while since I've done it, because I absolutely hate iTunes and how Apple locks your music to your computer, but if you go to your current iTunes music folder you should be able to find the music files with all of their names a seemingly random string of letters that I wasn't able to decipher.
But, if you go into iTunes, there is an option in the menu to convert all of your music into mp3's. It is either in the tools, properties, or one of the other menu tabs. It has been at least 3 years since I've done this on mine, and I haven't bought music through iTunes since then. When I get home, I will let you know the full path to find the option.
Once you convert your music to mp3's, you should be able to drag-and-drop the files onto your phone like the rest of your music. I am unsure how, or if this will get the album covers as well, because I have never cared about that myself to find out.