Can an OEM package be converted back to a .cab file?
 

I recently installed a Kelvin C&S and was wondering if there was a way to install additional OEM packages without having to recook the ROM? I was thinking that if I could convert an OEM package to a .cab file, I could easily install additional programs without losing existing customizations.

Sure but you'd have to do it manually. Get WinCE Cab Manager and import the rgu for the reg entries, add the files and create shortcuts (if any) and there you go.

There's no automated solution if that's what you were asking.

Most OEMs were created from freely available .cab files - you might try searching the internet for any OEM you want as a .cab.