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. But there might be some things you can do:
If you are operating within interactive Workspaces you might find the scratch space useful.
Or, 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.