as far as I'm aware, message text is not possible in S3M files?
In ST3 you can view sample data as text (don't remember the key combo) which looks a lot like IT's message editor. So, if you just load a text file in a sample slot, you have message text in an .S3M. And you can have as many as there are sample slots! :) If you have Purple Motion's modules, atleast zak.s3m has the legendary money.txt file with an evil pyramid scheme in sample slot #31.