You need a classic driver, with *.inf file and usually one *.sys and none up to several *.dll files. A setup file(*.exe or *.msi) will not work.
This driver files package must be in a folder, having the name of the *.inf file without the .inf extention.
Copy this folder into the <BaseDir>\lib\drivers\x86\win7 directory (create if not existant).
Important: You must flag that this driver is for installation.
Add an empty file <the driver's inf file>.install into the driver folder.
That may look like this:
The build will copy the package into <BaseDir>\output\wim-boot\drivers directory.
During boot of the PE the driver will be installed.
When for some reason you do not want to add a driver, just delete the flag .install file.
Create it again when you want to continue using the driver.