You don't actually want to do what you're asking - that would mean that no page on your site would 404 (if any missing page redirected to the root).
Better:
Redirect individual pages using:
RedirectPermanent oldpage.html newpage.html
Redirect sections using RedirectMatch or RewriteRule. Then set up a way of tracking 404 errors - either by checking your log files or by doing something like this to track them in google analytics:
http://analytics.blogspot.com/2006/09/tip-tracking-404-pages.html
Then, when you have a list of pages you missed or forgot, you can add redirections for those pages as well.
I hope that makes sense.