That seems to be the solution for me. As access  urls could only be set  up on the
webapp level, it seems that there is no possibility to set http://aaa.sss.com/EN
to be accessed via http://newaccess.sss.com, but http://newaccess.sss.com/EN
must also be OK.

You can create a forwarding. To do so conduct the following:

Open IIS
Right click Web Sites
Select New
Select Web Site
Select Next
Type in the description for the Redirection (ex: Redirect to
http://aaa.sss.com/en" target="_blank" rel="nofollow">http://aaa.sss.com/en)
Select All Unassigned
Select Port 80
Enter the host header http://newaccess.sss.com" target="_blank" rel="nofollow">http://newaccess.sss.com
Select NEXT
Enter c:\inetpub\wwwroot (leave allow anonymous access  checked)
Go with READ checked (accepting the defaults)
Click Finish
Right click on the newly created Redirect to http:/aaa.sss.com/en
Select PROPERTIES
Select HOME DIRECTORY
Select A REDIRECTION TO A URL
Type in the URL to http://aaa.sss.com/en" target="_blank" rel="nofollow">http://aaa.sss.com/en
Check the box THE EXACT URL ENTERED ABOVE
Select APPLY
Select OK.

Try to access http://newaccess.sss.com" target="_blank" rel="nofollow">http://newaccess.sss.com and it should redirect to
http://aaa.sss.com/en" target="_blank" rel="nofollow">http://aaa.sss.com/en.

You can setup an alternate access  Mapping to map aaa.sss.com to
newaccess.sss.com, but you won't be able to map a subsite address directly to a
root URL. The closest you can come would be that http://aaa.sss.com/EN would be
mapped to http://newaccess.sss.com/EN" target="_blank" rel="nofollow">http://newaccess.sss.com/EN, but not to http://newaccess.sss.com." target="_blank" rel="nofollow">http://newaccess.sss.com. The
only way I know of to achieve a vanity URL like http://newaccess.sss.com is to
create an IIS website at that address and use a redirect page to send the user
to the site at http://aaa.sss.com/en. You can't use AAMs to map subsites to root
URLs.

Redirecting the user is not the preferred approach, address bar should not be
changed. But I got the point thanks a lot.