Yes it is possible, you can change the content dynamically, like this.
public static string name = "Name from text field etc.";
public static int age = "Age from text field etc.";
public static int phone = "Phone from text field etc.";
public static string json = "{\"name\":\""+name+"\",\"age\":"+age+",\"phone\":"+phone+"}";
Here name, age and phone values are saved as you provide in text field.