rewrite URL to add www?

vmills

The Rewrite base for zenphoto is /portfolio. How can I modify the htaccess to rewrite mysite.com/portfolio to www.mysite.com/portfolio. I do this on the site's main htaccess file with this code

RewriteCond %{HTTP_HOST} !^www\.mysite\.com [NC]
RewriteRule ^(.*)$ http://www.mysite.com/$1 [L,R=301]

but it doesn't work in the portfolio directory when I add it to the htaccess file there.