^^ Free roam is not possible once you finish the game - you have to start a new game .. however all your weapons, mods, level will remain intact and the zombies will be also on th same level like your's ..
You can go back to the resort and finish your pending missions though you need to a find a safe house / boat like place with a map to go into main locations - but this will be disabled when you will be near the ending of the game ( though coming to the final stage takes lots of time if you are going to complete all the side quests ) - the game will warn you about this.
You can go back to the resort and finish your pending missions though you need to a find a safe house / boat like place with a map to go into main locations - but this will be disabled when you will be near the ending of the game ( though coming to the final stage takes lots of time if you are going to complete all the side quests ) - the game will warn you about this.