The word “saint” comes from the Latin word “sanctus” which means “holy”. This word can be applied to the members of the Church in general to denote the sanctity of the universal Church. But mostly, those who have this attached to their names are those who are in heaven. Angels are called saints because they are holy and are in heaven.
As for men becoming angels later, it is not possible. Angels are created different than men. Even when one has been transformed their very nature is different from that of the Angels.