>> and add usersname CNAME mydomain.com.
There no longer is such thing as CNAME in DNS. So don't use CNAME if at all pssible. Do a search using the search keyword: CNAME in this forum under my username to see why.
>> Is there any other way to do this quickly??
You can enable wildcard in BIND. For Apache, you can check mod_vhost_alias (http://httpd.apache.org/docs/mod/mod_vhost_alias.html). Or you can use include (http://httpd.apache.org/docs/mod/core.html#include).
If you don't have a reliable DNS, just don't do all this. If you don't have more than several thousands of username.yourdomain.com, don't do this either.