We can park that old domian name on top of the name you curently have hosted with us, and both will resolve to the current hosted account. They cannot go to separate sites with our hosting plan.
If you need the old domain name to go to it's own site, then you will either need to purchase another hosting plan, or a reseller plan.
If you want the old domain name parked on top of the current one, AND resolve to the same site, then open a help desk ticket. We will park it on top of your hosted name for free.