This week's book giveaway is in the Testing forum. We're giving away four copies of The Way of the Web Tester: A Beginner's Guide to Automating Tests and have Jonathan Rasmusson on-line! See this thread for details.
Yes, you can override ActionServlet. Make sure to configure the same in web.xml. Note: I have seen people often overriding ActionServlet do 'common things', most of the time the 'common things' can be implemented in Servlet filters.