Solution:
You can try something like that. I hope that help you.
RewriteEngine On
RewriteCond %{REQUEST_URI} ^/index\.php$
RewriteCond %{QUERY_STRING} ^slug=(.*)$
RewriteRule ^(.*)$ http://www.mywebsite.com/page/%1.html [R=302,L]
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPres