The only way i know to add a driver as a program is, adding to Post-Setup.
The other way to add a driver as inf, is to extract in the machine (when is necessary, as Lenovo an others, i think hp let us extract the drivers to a folder by running the exe installer), when is not possible to extract the driver, you have to extract the driver from the machine, go to properties' device, look for the inf's name, the sys, dll and other files name and location that could be necessary for the device and copy all of them to a folder.