Raw *.aac files are just that, they are raw AAC audio without any container. See, there are many containers out there and these containers can hold certain types of audio and video data. AAC audio is normally wrapped in a mpeg-4 container to make life easier. mpeg-4 AAC audio commonly had three types of filenames: *.m4a (used commonly by iTunes, Apple pretty much set this standard up), *.mp4 (commonly used by other, less known applications), and *.m4p (used only by Apple to show that your content was purchased from the iTunes Store). You can freely go from a *.mp4 to *.m4a by renaming your files, there is no need to convert since they are essentially the same thing.
You cannot simply convert a *.m4p file though as it contains DRM and prevents you from doing so. You will have to burn that to a audio CD and rip it as a different format. *.aac files can be made into *.m4a/*.mp4 files by taking the AAC audio and manually putting it in a mpeg-4 container. This can be done with many different applications. All of those formats are essentially the same thing though but their filenames distinguish them for different uses.