Upload a list of registered users to existing groups

This is a workaround of my latter question.

Is it possible for the administrator to:
*Upload from a .csv file (of e.g. just the username & email) of already registered members

*Add them to another existing group without a new registration procedure.

The point is that every new required registration loose members and the users have already said yes expecting to be directed to the other (sub)groups.

- any idea forward or known plugins that can do this is most appreceated!