ISPConfig can only act as a domain if you designate it ONLY as a domain server. So if you want to have both domain forwarding AND a server you will need two separate servers with two different IP's.
n1kol1s has a good suggestion. This is an easy way to redirect people to your new site Or you can get domain forwarding from a service. It is only a couple dollars per year and a more permanant solution.
1and1 cheap domain forwarding
also 1and1 automatically forwards sites with and without "www"