but a webhost might require use of their nameservers during addon creation (for security reasons).
obscure issue: if you use external email with your addon domain, people sending email to that domain from the same server might have a problem. for example, using microsoft's free email with your addon domain ( http://domains.live.com ).
Just need to insure you go to WHM / Tweak Settings / Allow Creation of Parked/Addon Domains that resolve to other servers...... and check that box.
As others stated make sure you add the record to your NameServers first and have fun
I don't have access to WHM as I am trying to use shared hosting with addon domains to avoid having a reseller account (since all the domains to be hosted are mine there isn't a security/privacy issue).