Create custom install.wim with WinPE

Fire up the WinPE ISO made earlier, make sure NOT TO BOOT the regular HDD or you will have to restart.

Now using the CLI you need to mount the system partition and create the image.

https://docs.microsoft.com/en-us/windows-hardware/manufacture/desktop/capture-images-of-hard-disk-partitions-using-dism

diskpart
list disk

DISKPART> list disk

Disk ###  Status         Size     Free     Dyn  Gpt
--------  -------------  -------  -------  ---  ---
Disk 0    Online          127 GB      0 B        *

select disk 0
list partition

DISKPART> list partition

  Partition ###  Type              Size     Offset
  -------------  ----------------  -------  -------
  Partition 1    Recovery           300 MB  1024 KB
  Partition 2    System             100 MB   451 MB
  Partition 3    Reserved            16 MB   551 MB
  Partition 4    Primary            126 GB   567 MB

select partition=4
assign letter=S
exit

Dism /Capture-Image /ImageFile:s:\win10sys.wim /CaptureDir:S:\ /Name:"Windows 10 v1709 Enterprise"

Now you need to extract that file from the VM, there are many ways so I leave that to you. Personally, I fired up a LiveISO and copied the file to a network share.

Note: You can just fire up the VM, the whole reason for not booting the HDD before doing this is it will setup the system just like the custom ISO would, but you need to capture the image BEFORE that happens. After you create win10sys.wim, you can just fire up the VM and copy the file from C:\win10sys.wim to your host computer via network share/whatever. It's also a good test of what an installed system would look like with the install.