Manually enroll users and set additional email address for verification code

I have a group of users which I'd like to manually enroll and add a second email address to. Within this group is a subset of users who have already enrolled, but have not set a second email address for verification code resets.

Using the 'Import enrollment data from CSV' option gives me the error 'header is not consistent'. However, I assume if it did work it is only used to set the primary email address, which is already done for these users. The CSV is UTF-8 and the simple, single user file I'm testing import against looks like this:

samAccountName,mail
first.last, firstlast@asi.com

How can i manually enroll multiple users with their main email addresses and also add a second email address to use as their verification code address?