Multiple prepaid accounts

I’m about to activate a second prepaid phone number, but I see no obvious way of doing this when I log in to my prepaid account.  Is there a way to do this? I’d like to log in to one account and manage both prepaid phones. The current prepaid activation page (https://activateprepaid.koodomobile.com/), doesn’t offer an option to use an existing account, I only see  section to set up a new account.  I don’t see “add a phone number” or something similar when I log in to my existing prepaid account.