Are you looking for hop flavor and aroma or bitterness? If you are looking for bitterness, you are a little too late (well I suppose you could look for hop extracts, but I'm not sure how those work). If you are looking to just get some flavor and aroma, then yes, you can just add hops to your carboy (this is dry hopping). When dry hopping, I will let the beer ferment out completely and transfer to a secondary... this is where I add the dry hops. In the case of a porter, I imagine East Kent Goldings or Fuggles would be nice.
EDIT: BTW, dry hopping a porter is a little unusual because the malt dominates... but I know some who have done it with good results...