If I understand right, then You have some static html files which You want to show directly, only if the file does not exist, You want to use Your CI installation.
You can use this .htaccess file, it’s not specially written for Yous case, but it will do serve any existing file, everything else is passed to the CI installation, it also takes out the index.php of the URL :
<IfModule mod_rewrite.c>
RewriteEngine On
# On some servers the base need to be set
RewriteBase /
# if file / folder does not exist
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
# Take out index.php
RewriteRule ^(.*)$ index.php/$1 [L]
</IfModule>