I think what you going to have to do here is use the GotFocus event and manuall open the dropdown.  Something like this:

 <ig:XamComboEditor ItemsSource="{Binding}"
                    DisplayMemberPath="Name"
                    OpenDropDownOnTyping="True"
                    CustomValueEnteredAction="Add"
                    GotFocus="XamComboEditor_GotFocus"  />

private void XamComboEditor_GotFocus(object sender, RoutedEventArgs e)
{
    ((Infragistics.Controls.Editors.XamComboEditor)sender).IsDropDownOpen = true;
}

This will pop open the dropdown as soon as the control gets focus, even if the user has not typed anything.

Devin