Short answer: Of course you can do all that, but it takes a lot of time.
Converting from a static site to a dynamic one can be extremely time consuming. Especially if you aren't sure where to start. If you do it one time ( to make just the sidebar dynamic ) you may as well do it right that one time, and make it all dynamic. Separate the header, navigation, footer, etc. Either way you'll still have to edit every single file on the site to be both php, and include the necessary files.
Judging from your questions, you seem pretty novice to any of this stuff. If that is the case, I suggest recreating your site with Drupal, Expression Engine, or (maybe) Wordpress. They're all cms frameworks, and they offer a pretty good jump start to beginners.
|