Can I use sweetened condensed milk caramel for caramel apples? If not, is there something i can add to the caramel to make it harden enough?

Best to find a recipe that uses sweetened condensed milk. Candymaking is a science and with hard sugar candies, the hardness is based on the temperature to which the sugar cooks. If you want a hard caramel, you would have to cook it to the hard crack stage (~300F).