You will need to purchase a porch deed and drop it onto your house. Deeds can be purchased at any housing marketplace (or at your cornerstone before the house is placed).
In order to add a porch, you must also pick up all the decorations from your house and garden. This can be done through the command /housepickup, however this command will only pick up decoration objects - not Merchants, Vaults, or Tradeskill Tools. You must remove these items yourself.