You’re better off working with IDs (id in a user json) - which never change for a user, as opposed to handles (screen_name) - As long as it’s just lists of IDs, there are no restrictions on exporting those.
For analysis, you can retrieve fresh user information with https://dev.twitter.com/rest/reference/get/users/lookup - This way any account that becomes private, or deleted, or any changes to profiles will be respected when you lookup a user.
I think Ads API handles lists of user ids, but i don’t know the API.