Yes, you can extend ActionServlet. If you do extend it, all you'd have to do is put the name of your subclass of ActionServlet in the web.xml file instead of ActionServlet.
However, once you study the Struts source code you will find that ActionServlet doesn't really do much. The bulk of the work is done by RequestProcessor. If you want to change how Struts works, you'll most likely want to extend RequestProcessor rather than ActionServlet.
If you extend request processor, just put the following entry in your struts-config.xml file: