Hushmail Business customers can set up more than one domain under their 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. This way, the subscription fees for all domains under your account will be processed as one charge, rather than billed separately.
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.