subdomains

[axc]

i have a domain hosted on hypermart.net
http://www.fotoradar.com
and would like to have multiple subdomains
like sales.fotoradar.com or abc.fotoradar.com
i understand is it not posible on hypermart, but I wonder
if maybe using the free dns server granitecanyon.com
I could change some record so this domains all point to the
same address www.fotoradar.com and then use a cgi script
to send the users to diferent sub directories on the same site
please help any advise is welcome
thanks