use my own template

Hi
I'm new to open cart and have a few questions.

1. I have my site already set up with the template I like. Is there a way to add open cart to it
without having to change/redo everything just add some code to each page?

2. Do I have to rename all my pages with .php or can I keep the .html extension?

Any help with this will be appreciated.

My coding abilities are limited to css and html