this is a really interesting question, at first view, yes! but i tried to add my gmail extension to the default installation and i wasn't sucessful. if you export your full profile as said in step2, it's not necessary to slipstream extensions on your installation because they're already installed, but... the interesting part it's to create a silent installation with custom ext. if any1 discovers a way to integrate them on the install please, let me know and i'll update the guide.