How do I update a nutch db?

[Paul M Lieberman]

I've created a db of roughly 250,000 entries from a few of our websites. 
 I did this with CrawlTool (depth 10).

How would I go about doing a nightly update to add more pages to the db?
I have looked high and low through the documentation, and have not been 
able to ferret this out.

TIA,

Paul Lieberman
American Psychological Association