Editing individual subsite htaccess for multisite installation

I was able to force browsers to always display www in the url of all site pages of a single site installation by adding the following to the htaccess file:
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\.mysite\.com$ [NC]
RewriteRule ^(.*) http://www.mysite.com/$1 [L,R]
This obviously doesn’t work for a multisite installation. Do you have any advice as to how I can accomplish this on a multisite?