You need to export the users from AEC and format them to work with CBSubs import.
File (single line of comma-separated user-ids)
This text file is simply a comma-separated list on a single line of the user ids for which the subscriptions should be created.
Example: 63,64,65,102
File (each line: userid,planid,status,subscription date)
This text file is multiple lines (one line per user) comma-separated list with following format: userid,planid,statusSingleChar,SubscriptionDateISO.
Status: A = Active, X = Expired, C= Unsubscribed/Cancelled.
Example: 63,1,A,2009-01-01 00:00:00
All users from a given CMS ACL group
Select ACL group
All users from a given subscription plan
Choose existing CBSubs plan
All these require plans to be created within CBSubs already. You can select the plan, plan status (active, deactive, cancelled), as well as the subscription start date after selecting your user import method.