You could not at a subdomain via a .htaccess file.
You need to modify the vhost configuration and add one more vhost.
Have a look at the documentation of your webserver how to do this.
There is a logical reason behind this (i don't know all webservers and there features, so correct me if i tell something wrong):
Lets look at a typical HTTP request(send by client):
GET /index.php HTTP/1.1
So, we have the following information in this request:
- the uri:
- the protocol version we understand:
- the Host (vhost) we want the uri from
On such a request, the webserver does a few lookups:
Host: first the server needs to know in which directory he should look, this is part of the vHost configuration
- the uri you are looking for
As the .htaccess file does belongs to a vhost (a.example.org), we have already behind the vhost decision.