The domains do not need to be in the same cpanel. If your server is configured correctly then just point the domains DNS settings (a record) to your server IP address and your config will take over and point the domain to your sub blogs.
I have domains from all over the place pointing to my multisite.
You can test this with a free domain name. You can get a free name from http://dot.tk
Do not use dot.tk for a main website, they have been known to take back their domains and park them when they get popular. But, it's a good tool for testing multiple domains. Register for one for free and point the A record to your static IP address. You can register as many as you want.