Can I use a post conditional statement with LINQ?

It sounds like you want the reverse of the null-coalescing operator, and there's nothing in either LINQ or the C# language to do that for you. You could create your own extension method to do that if you wanted though:

public T? NullIfDefault<T>(this T? value) where T : struct
{
    if (value == null)
    {
        return value;
    }
    return EqualityComparer<T>.Default.Equals(default(T), value.Value)
        ? null : value;
}

Then:

double? myVar = myList.Sum(x => x.MyItem).NullIfDefault();

(There are various other options for how you code NullIfDefault, e.g. using Nullable<T>.GetValueOrDefault() - you might want to play around to find the form you like most.)