from http://blog.uvm.edu/jgm/2011/03/31/winpe-loading-drivers-to-a-live-instance/

1. Get you driver, load it to removable media, connect the media to your
running WinPE instance.

2. “cd” to the driver directory

3. Run “drvload [path to inf file]“

4. If you just loaded a network driver, run “wpeutil initializeNetwork”.

Done!