For single domain hosting, add-on is not been upgraded to your existing account. Only for multiple domains add-on is available to your existing account. It is possible for both Linux and windows.
For example, if you have choosen Linux website. Follow the steps to purchase an add-on:
1) Go to customer account --> Login
2) Order ----> My invoices
3) Select 'Upgrade' option below hosting.
Note: Use search option in case for multiple hosting.
4) Choose your plan as Linux - 3 Unlimited or Linux - 10 unlimited
1) Linux 3 Unlimited - You will get 2 add-ons and 1 domain.
2) Linux 10 Unlimited - You will get 9 add-ons and 1 domain.
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