Signing students up with their usernames?

I have a unique situation where my students had to sign-up for Khan before I created my class in order to complete a county online learning unit.  I have all my students usernames.  I was wondering if there was a "workaround" so that I could enter my students into my class using their usernames instead of inviting them by email or having them add me as a teacher.  As usernames should be unique anyway.  It would just be nice to enter them in my class myself instead of trying go get student to do one more thing at this time.  Please let me know if you have any ideas.