unfortunately, there is currently no option for an ML iframe, you might need to enhance the module yourself or use another solution.

if all subdomains are routed to the same IIS web site, you simply need to create a new parent portal and use "parent.mydomain.com" as portal address instead of www.mydomain.com/parent as for a child portal (if www.mydomain.com is the address of your portal and parent the name of the new subportal. To create a parent portal login to your dotnetnuke installation, go to portals in host menu and click add. enter details and create.