CNAME Record in .htaccess file: correct format?

Question

In my .htaccess file, I have set up several A records in the following structure:

RewriteCond %{HTTP_HOST} ^domain.com$ [OR]
RewriteCond %{HTTP_HOST} ^www.domain.com$
RewriteRule ^substring\/substring\/etc?$ 
"http\:\/\/domain\.com\/newsubstring\/etc\/" [R=301,L]

Is is possible to easily set up a CNAME record in the same fashion and put it in my .htaccess file? Unfortunately, my dumb DNS (namecheap) doesn't let me change CNAME records because my site actually has hosting and isn't just a parked domain.

(I actually just want to set up a CNAME for www, since at the moment www.domain.com doesn't work for me, and only domain.com -- I'm going to point the former to the latter.)