If you have the option to Addon domains in your cpanel, then at the domain registrar, you would need to set the nameservers for that domain name to the same ones you are using with us now, then create the Addon domain.
Go to the domain registrar and change the nameservers. After changing the nameservers, go to your cpanel http://www.SITENAME.COM/cpanel/ OR cpanel.SITENAME.COM and create the Addon domain by clicking on Addon Domains.
Helpful Links:
cPanel User Documentation
https://documentation.cpanel.net/display/ALD/cPanel+User+Documentation
cPanel Tutorials
https://www.youtube.com/playlist?list=PLZk46idJS6s54hAX8K79_AY8brgBwPtHd
cPanel & WHM Documentation (Dedicated Server Clients & Resellers)
https://documentation.cpanel.net
WebHost Manager - WHM User Guide (Dedicated Server Clients & Resellers)
https://documentation.cpanel.net/pages/viewpage.action?pageId=1507786
