Unfortunately, I think you have to replace the entire template:
From MSDN: http://msdn.microsoft.com/en-us/library/aa970773.aspx
Controls in Windows Presentation
Foundation (WPF) have a
ControlTemplate that contains the
visual tree of that control. You can
change the structure and appearance of
a control by modifying the
ControlTemplate of that control. There
is no way to replace only part of the
visual tree of a control; to change
the visual tree of a control you must
set the Template property of the
control to its new and complete
ControlTemplate.