Hello,
Changing the document root & redirecting a domain, both are different procedures & none of them can be done through WHM. The result that you are expecting cannot be obtained from either of the options. The best way to get this working is to use URL MASKING. URL Masking means that the visitors to the domain name will be forwarded to the destination website, however the URL in the browser address bar will remain the same. Following is an example for URL Masking.
Code:
<frameset rows="100%">
<frame src="http://www.main.mydomain.com/">
</frameset>
<noframes>
Please follow <a href="http://www.mydomain.com/">link</a>!
</noframes>
You have to use the code in the index file of the main domain.