Jam and Marmalade setting
Yes you can. You do want to make sure you have given your marmalade a couple of weeks to set completely before deciding to remake with additional pectin. 

In the US, pectin found on grocery store shelves generally contain a sheet with basic recipes, as well as instructions on how to remake your cooked jam/jelly/marmalade if it doesn't set properly. 

Here are additional options from The National Center for Home Food Preservation:


  Making Jams and Jellies
  
  Remaking Soft Jellies
  
  Measure jelly to be recooked. Work with no more than 4 to 6 cups at a
  time.
  
  Please read Using Boiling Water Canners before beginning. If this is
  your first time canning, it is recommended that you read Principles of
  Home Canning.
  
  To Remake With Powdered Pectin
  
  For each quart of jelly, mix ¼ cup sugar, ½ cup water, 2 tablespoons
  bottled lemon juice, and 4 teaspoons powdered pectin. Bring to a boil
  while stirring. Add jelly and bring to a rolling boil over high heat,
  stirring constantly. Boil hard ½ minute. Remove from heat, quickly
  skim foam off jelly, and fill sterile jars, leaving ¼-inch headspace.
  Adjust new lids and process as recommended in Table 1. For more
  information on how to sterilize jars see "Sterilization of Empty
  Jars".
  
  To Remake With Liquid Pectin
  
  For each quart of jelly, measure ¾ cup sugar, 2 tablespoons bottled
  lemon juice, and 2 tablespoons liquid pectin. Bring jelly only to boil
  over high heat, while stirring. Remove from heat and quickly add the
  sugar, lemon juice, and pectin. Bring to a full rolling boil, stirring
  constantly. Boil hard for 1 minute. Quickly skim off foam and fill
  sterile jars, leaving ¼-inch headspace. Adjust new lids and process as
  recommended in Table 1.
  
  To Remake Without Added Pectin
  
  For each quart of jelly, add 2 tablespoons bottled lemon juice. Heat
  to boiling and boil for 3 to 4 minutes. Use one of the tests described
  in Testing Jelly without Added Pectin to determine jelly doneness.
  Remove from heat, quickly skim off foam, and fill sterile jars,
  leaving ¼-inch headspace. Adjust new lids and process as recommended in Table 1..