Hushmail Business customers can setup more than one domain under their customer account. In fact we encourage customers who wish to have more than one domain to let us know so we can consolidate the domains into one account. The benefits of this are that customers receive one monthly charge for all their services (as opposed to one charge per domain).
If you wish to enable another domain please see the Making changes to your Hushmail Business services article. There are no additional costs to add additional domains besides the cost of new users on those domains.