You may be able to do it if you log onto your primary account and go to "settings" (small cog icon) then "Settings" > "Accounts" > Add another Mailbox > follow the prompts for setting them up.
This will allow you to have your sub accounts accessed via the master account. You will be able to send from the sub accounts by using the dropdown box when composing emails.
The easiest way however is to use an email client such as Windows Live Mail or Thunderbird. If you do that you can set it up to access as many email accounts as you want.