You can add unlimited addon domains to your iisnet-networks account. Firstly please ensure that the name servers are pointing to the nameservers that is sent on iisnet-networks welcome email.
Then login to your cPanel account and select 'Addon Domains' under the 'Domains' menu.
Complete the form as follows:
New Domain Name: the domain you wish to add
Subdomain/FTP Username: Whatever you put in the this field will be the folder that is created under your primary account. It will also be the default username for mail and FTP.
Password: your password
The change in nameservers may take up to 48 hours.
If you would like to purchase a new domain name you can do so at the top of your cPanel account, just click 'Domain Registration'.