Is it possible to clone a ValueType?

I don't know, if I have totally misunderstood the question.
Are you trying to do this?

public static void Main()
{
    List<ValueType> values = new List<ValueType> {3, DateTime.Now, 23.4M};
    DuplicateLastItem(values);

    Console.WriteLine(values[2]);
    Console.WriteLine(values[3]);
    values[3] = 20;
    Console.WriteLine(values[2]);
    Console.WriteLine(values[3]);
}

static void DuplicateLastItem(List<ValueType> values2)
{
    values2.Add(values2[values2.Count - 1]);
}

Every assignment of a valuetype is by definition a clone.

Edit:

When boxing a valuetype a copy of your valuetype will be contained in an instance of a ReferenceType.

Depending of the Cloning method, I don't foresee any differences.

You can use a hack using Convert.ChangeType:

object x = 1;
var type = x.GetType();
var clone = Convert.ChangeType(x, type);

// Make sure it works
Assert.AreNotSame(x, clone);

The result is copy of the value boxed in new object.