You can add device drivers to an existing Windows PE device driver set or you can create a new driver set.
Each driver set has a DrvSet.ini file in the root of the directory. The Toolkit Windows PE build process reads this file and uses the fields Windows Version, Creation Date, and Creation Time to determine which drivers are the most recent, and therefore will be installed. By modifying the directory structure and creating a new DrvSet.ini file, you can either add drivers to existing driver sets, or you can create a new driver set.