If you want to extend your hosting account you can upgrade the options of your account. another way to extend your account is using an addon.
Login to the client area, then click on Services -> My Services
Select for which product you want to add an option from the list, then click on Upgrade/Downgrade Options (if present for your plan) in the right sidebar.
Now select the options you’re interested in or eliminate the ones you’re no more interested in and follow the steps. Activation may require up to 24 hours as this is a manual process.