You can definitely add a phone number to an account that already exists.  In the case of adding a phone number to a Full Service Account, the phone number will be setup as a virtual phone number which means that by default it will simply serve as an alias to this Full Service Line account.  In the case of adding a phone number to a Forwarding or Fax Only account, the phone number will be setup as the specified type.  You are able to add Forwarding only phone numbers to Fax Only accounts.  You will then be able to manage the phone numbers from the one account.  You can do this simply by logging into your Reseller Control panel and clicking on Accounts -> Then continue to click on the applicable account in question.  You will then see a section to "Add Phone Numbers to Account".