Dynamically adding S4 class slots

I'm new to S4 objects and therefor I have a big doubt. If I want to dynamically add slots to my class, should I be using another type system?

As I understand S4 enforces class definition at writing time. If I want to dynamically decorate my object with new properties, shoud I be using RC or S3?

On another line of reasoning, is class union the right technique to decorate my objects?