If you need to add drivers to a Windows install USB then do this:
1. Copy the install.wim file from the USB stick to your local drive.
(optional, to speed up the process)
2. Run the following command to mount the Windows image file.
Assumes that you put the file in C:\temp and have created a C:\mount folder
Dism /mount-wim /wimfile:C:\temp\install.wim /index:1 /mountdir:c:\mount
3. Add the drivers to the image.
Assumes that the drivers are located in c:\temp\x64
Dism /Add-Driver /Image:"c:\mount" /Driver:"C:\temp\x64" /Recurse
4. Unmount the Windows image file
dism /unmount-wim /Mountdir:c:\mount /commit
5. Copy the install.wim file back to the USB stick