I want to retrieve all active users in the system and to insert them into a DB t

This probably isn't quite as slick a solution as your were hoping for but it's probably how I'd do it....

Getting a list of all active users: forum.appian.com/.../f-107358

Saving into a DB table:
1. Create a CDT of the user fields you'd like to save
2. Create a new process model that will run on a schedule to pull down all your active users using the method above (timer start node)
3. Map the user data to your new CDT (maybe using fn!group to get your users details)
4. Then use the Write to DB smart service to save the data.

Thanks a million Mike; that helps!