It's not currently possible to refresh a Dataset in a running session (or to mount an existing Snapshot after the session starts). Only the Snapshots specified when you launch the session will be available. Since Snapshots are read-only once created, and not accessible across sessions until then, they are not really suited for truly streaming data workflows.
If new files are not actually added very often, but you want to have a long-running script A "watching" for them to do some processing, you might find the Domino API useful. Using the API you can check the list of Dataset Snapshots, and if there is a new one, launch a separate Job via an API call in your code to do the processing on the new snapshot.