URL rewriting for WordPress Network (Multisite) subsite

This is straightforward if I understand the question properly. Use the “WordPress MU Domain Mapping” plugin:

http://wordpress.org/extend/plugins/wordpress-mu-domain-mapping/

Update

I think something like the following should work in your .htaccess:

RewriteCond %{HTTP_HOST} www.exampleurl.com
RewriteCond %{REQUEST_URI} !^/testsite
RewriteRule ^(.*)$ testsite/$1 [L]

This will return the contents of the folder “testsite” for “www.exampleurl.com”. And if you have:

www.exampleurl.com/myoldsite/

then I think it would return the contents of:

/testsite/myoldsite/

So I am guessing if you but WordPress in that folder it should work. You might also need to map www.exampleurl.com to blogs.exampleurl.com using the domain mapping plugin.