How to extend core drupal modules - Newbie question

I am newbie to Drupal. First, I thank the Drupal developers for providing this excellent software.

As everyone, I am trying to understand the basic way of doing things in Drupal. The is my first problem. I would like to use the Poll module from Drupal, but I just want to make few changes to the existing poll module. Like everyone, I do NOT want to copy the current module code and create a new module and make changes to it. I searched the forum and the best I could sample I could find is 'nodeapi_example.module'. But this module really implements some of the standard hooks, it doesn't really extend an existing mdoule and override few functions. My expectation was, just override the methods which I would like to change. Could somebody explain the concept behind extending a module in Drupal? Please forgive me, if this is already answered in forum. I did search for 'extend module' and I couldn't find any useful answer.

If lets say I want to add one more attribute to the Poll, what is the best way to do this?
1) Should I add the new column to the core poll table or do I create a separate new table and add the new column?
2) if I have to add a new table, then how do I modify the existing sqls to do a join with the core poll table?

Any help is appreciated, Thanks