This you can do it in your domain name control panel. There you can see the Web Forwarding option and there you have to put the Domain name which you have to forward.
Like say your default domain name is www.abc.com and you have to forward it to www.abc.org thn go to the control panel of the domain www.abc.com and there go in the settings as said above and modify with then .org name and u r forwarding is done.