Sounds about right to me, probably the easiest way too. You really wouldn't have alot of downtime really you just need to change your dns or even add it as a third/fourth nameserver until they resolve.
WHM and it's DNS service will prevent you from creating another account with a domain that is already on the server. If you do want to keep up the site while you are transferring it, I would suggest creating a new account with a different domain (some random domain that doesn't actually exist on your server). Once you have it all setup, delete the addon domain, and edit the account through WHM and change the main domain for it to the actual domain. Although cPanel does not recommend changing the main domain, I have done it in the past and have not experienced any issues (at least ones that I could see), and this should result in minimal downtime.