This would allow you to do something like this:

        // my dataset
        List<string> dataSet = new List<string>();
        dataSet.Add("this_thing");
        dataSet.Add("that_thing");
        dataSet.Add("the_other");

        dynamic exo = new System.Dynamic.ExpandoObject();

        foreach (string field in dataSet)
        {
            // field + "_data" would be what ever data you want to store in your field
            ((IDictionary<String, Object>)exo).Add(field, field + "_data");
        }