It is possible to add new mime types in .htaccess files.
Add a .htaccess file in your webroot, ex. /home/username/public_html/yourdomainname/.htaccess , then add a line like
AddType application/rss+xml .rss to add the new mime type application/xml+rss for .rss files for that domain.
And isn't it the handler your suppose to change not the mime type? Since asp scripts run on the server to generate html(xhtml) code that should be delivered with the mime type text/html(application/xml+xhtml)?