It seems to be pretty cool feature and some of our customers are asking for the feature.
How do I add this functionality to a customer account, which is already setup? I couldn't find anyway to do this upgrade or changing the packages or anything. Is there a workaround other than deleting and adding the account back with addon domains functionality
On WHM go to "Modify an Account"
Edit the account to modify and you'll see two new items to modify
"Max Parked Domains" and "Max Addon Domains"
BTW, taking this a bit off-topic, I hate what Cpanel did with this, they didn't gave us the option of remove that option for those wo don't want to offer multi-domain hosting.
Off course that I can keep that in 0 but still I got a lot of question about this "new feature"