How to add a Membership Member Level when importing users by CSV?

I'm adding around 3000 users to a WordPress site from a legacy database, and have converted all my data to CSV files, which import fine using the Import Users from CSV plugin.

But I also need to add the "Members" level to all my new users. As the import plugin supports custom meta fields, can I add my new members to the member level that way? Or is there a script I can run to automatically add the Members level to all users with a Subscriber role (added in the CSV)?

Or do I have to do it all through the interface?