You can schedule server-side scripts by setting up cron jobs with cURL or wget. See this thread.
Now, how would you know when the DB has been changed by the external DB? If your external service alters the DB directly, how do you expect to be notified? Can the external service be altered to also call the script when it makes the DB change? An API is just an API–it does nothing unless called (whether called by your external service, or a cron job, or a user).
The difference would be trivial whether the DB is the local one or an external one. From the DreamFactory instance’s perspective, it’s just the same operation on a different service name.