You can add one or more domains in a single host or in a single server that you host. These are Addon Domains. For the server which is already hosting your Primary domain, addon domains are no different than subdomains. But these addon domains portray themselves as a single website, to the web-world. Though these cannot have separate cpanel they can have separate ftp accounts & email accounts, sub-domains, redirects, fantastico installations and all, which a normal domain name can have. This can be done within your allotted Webspace and Bandwidth for the cpanel. And you can have different web contents in them. Without a separate domain(the domain you gonna make addon domain), you may not be able to create Addon Domain with your existing Primary domain. Also if the new domain's DNS is not propagated, it is not possible to create an Addon with it.
Creating an Addon Domain can be done in your cpanel.