Yes, you can purchase ‘Have It All’ while you are on board, although buying it before your cruise is usually the best way to get the most overall value.
After you are on board, the package can be added by visiting Guest Services or speaking with onboard staff, who can confirm eligibility and complete the purchase directly to your shipboard account. While pre-cruise purchase typically offers the strongest overall value, this onboard option allows you to upgrade mid-voyage if you decide you would like the added inclusions.