If i understand you right, you need DXDraw Surface with another BPP than current screen mode in windows has ??
Simply set the needed bpp up in the DXDRAW object property page. If your app runs in fullscreen mode - the GUI resolution is automatically changed to that one of DXDraw. Maybe there's code in your program that setup DXDraw with the windows bpp ?

Under Delphix/samples/graphic/basic is an example program which use 8bpp / windowed mode.