Are these emails meant for notifications, or for logins? If for logins, I believe thats possible with the logins.csv file. If it's to put in their profile for notifications, I con't think that can be done with the CSV files.
Sorry I don't have a good solution for this one (I know it could be done with API, but you'd need to have someone who's good at scripting create something to do it). I'm curious as to the use case for adding a second email to Canvas though, Does your institution assign multiple email addresses to students or are you thinking about adding both an institution email and a personal email address for your users?