The technique depends on how the message you want to attached the CSV file has been created.
If you used commands like GroupWise.NewMail() to create a visible email message, you would use GroupWise.ItemAttachmentAdd(). See the Formativ Language Guide for details.
If you used the Object API to create a message, you would use the message properties Attachments collection. See http://developer.novell.com/ndk/doc/gwobjapi/gwobjenu/data/hej0zz9f.html#hej0zz9f for details.
I hope this helps.