no, you cannot.
NACS uses its own database for storing session, protocol data, etc.
It's not possible to use external database (e.g. Oracle) for that purpose.
But if you want to use connection to external DB (E.g. for your own authentication), you can write servlet which requests username/password from a user and then send store/retrieve this data from DB. This is just an example, but it should help somehow :)
How to add database component in Nokia Activ Server 2.1 ?