how to add expiration date to a question (post)

Question

I want add a attribute: "Expiration Date" to each post, which means I need to add a column in qa_posts table named "expiredate".

Then I need to insert a new post into the table with this "expiredate" (I already have a datepicker plugin of JQuery on the ask page), when clicking "Ask the Question" button.

How can I do that? What php file should I modify? Or is it possible that I just overwrite inserting method without changing the core code?

Thanks !