Comments by Ivan Leonenko (Top 1 by date)

You can still make custom control and apply styles and control templates to them, if you inherit from ComboBox http://msdn.microsoft.com/en-us/library/ms752094(v=vs.85).aspx . Or you can use the same approach the default style & control template for ComboBox using, IsEditable does only this:
<Trigger Property="IsEditable" Value="true">
<setter property="IsTabStop" value="false"></setter>
<setter targetname="PART_EditableTextBox" property="Visibility" value="Visible"></setter>
<setter targetname="ContentSite" property="Visibility" value="Hidden"></setter>
</Trigger>