Convert mp3 to mp4 for Audiobook

Hi,

I have an mp3 file of Eddie Murphy's "Raw" show. I want this to appear in the Audiobooks tab, seperate from my music. I have read other posts saying you can convert an mp4 file into an m4b format (audiobook format). As my file is in mp3, how can I convert it from this into mp4 so I can have it displayed as an audiobook?

Thanks,

darrkaye