The Web panel allows you to add plans to an existing account. As far as I know, the space and bandwidth allowances are global for the account, so if you had (say) 3 Level 1 plans under the one account, the total space and bandwidth available would be 60GB and 3 TB respectively.
The Web panel also allows you to merge accounts, but it is probably easier just to use the one account and add the required plans to it.
By the way; If you are a new customer, it should be OK to use a promo code for the first account/plan.
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