There are probably a few ways to do this. You could manage your own ExpandableFieldRecord by setting the IsExpandable property to false and expanding it programmatically (or through Binding).
Also check out the following section in the documentation:
Working with xamDataGrid, xamDataCarousel, and xamDataPresenter Styling Points<http://help.infragistics.com/Help/NetAdvantage/WPF/2008.1/CLR3.X/html/WPF_Working_with_xamDataGrid_xamDataCarousel_xamDataPresenter_Styling_Points.html>
This section goes into detail how to stylize parts of the data grid chrome.