I manage a multi-class site; in fact, it is for all classes from our high school together. Some of the individual classes already have Class Creator sites set up. Can I import the lists that are already set up on those sites into mine, without having to convert back to Excel .csv files? If not, do I need to get their site admin to export their lists to Excel? Thanks.

Unfortunately there is currently no direct import mechanism to pull names from one site into another. So yes, you need to have the Admins on those sites export their class list, copy and paste those names into the Excel import template, and then upload to your site.