If you have two accounts, and only want one account that includes all the Pobox addresses, go to the Remove Addresses page and use the "Move to Another User" button. If you want to be able to forward mail on the consolidated account by Pobox address, it will need to be a Pobox Plus or Mailstore account.
If you have multiple accounts, and you want to consolidate your bills or if you have one account, but want another non-Pobox customer to be responsible for paying the bill, use our help page How to combine accounts and change billing contacts.