Well, adding a new screen mode preset is quite complicated. It requires lot of changes in many files and it always takes me lot of time.
You can switch to your preferred screen mode manually, adjust the icon positions of your desktop, etc. but the boot picture cannot be resized correctly. This is because all the boot pictures were painted manually specially for the given screenmode, you know. Better disable the boot picture completely (check this
thread). Or you can import your own, just play with the AmiKit:Prefs/ShowAmiga96_Prefs.