I don't recall anyone posting with your exact situation. I'd suggest submitting a ticket with the question.
Since you're already a registered customer and have a panel account and are "just" looking to add a hosting account to it, it's probably something that they'd have to do manually if they do let you do it.
Then again, you could always just sign up for a new account and hope for the best. You'd probably run into problems if you used the same credit card attached to your current domain registration-only account
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