Your CDI plan can contain up to 3 fully hosted domains, so there's no problem with adding your new domain. Just go ahead and do it.
With the plan you get 1 free domain registration, and if you haven't used it yet, then you can use it any time you want, or save it for as long as you want. Once you use the free registration, it's used. If you stop using that domain, you don't get a new free registration.
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