How can I copy the drivers provided by Apple when installing Bootcamp on an external drive?

The dism command you are referring to was originally posted as part of this answer. Colin William posted 2 responses which explain how and when to use a Windows laptop to execute the command.

I originally posted that the dism command should be executed while running in the virtual machine. However, after reading Colin William's posts, I decided for this answer to build on his experience and post how to use the dism command after shutting down the virtual machine. The steps are given below.

1. Julius Pargac's procedure calls for copying the Window Support Software to a MS-DOS (FAT) formatted 4 GB or larger flash drive. Instead, use a ExFAT formatted 16 GB or larger flash drive. Additionally, copy all the files from the Windows 10 ISO to this flash drive. The result should appear similar to what is shown below.

   

2. Proceed with Julius Pargac's procedure until after you have closed VirtualBox. This is immediately after you have manually shutdown the virtual machine.

3. Eject and unplug the external drive where windows is installed. Plug in the flash drive. Restart the Mac and immediately hold down the option key until the Mac Startup Manager icons appear. Select the external drive icon labeled EFI Boot.

4. The first window to appear should be similar to the one shown below.

   

   Proceed as one would for a GUI installation of Windows. When a window similar to the one shown below appears, press the shift+F10 key combination.

   

   The result should be the appearance of the Command Prompt window shown below.

   

5. Plug in the external drive where Windows is installed.

6. Enter the command given below. This will list the volumes.

   echo list volume | diskpart
   

   From the output, determine the drive letters for the flash drive and the external drive where Windows is installed. I found letter C: for the flash drive and letter D: for the external drive where Windows is installed. If you have different drive letters, then make the appropriate substitutions when entering the command given below. This command copies the Windows Support Software drivers to the Windows 10 drivers store.

   dism /image:d:\ /add-driver /driver:c:\$WinPEDriver$ /recurse /forceunsigned
   

   Note: If a driver meant to be installed after Windows has finished installing, then an error message will be displayed. You can ignore such messages.

7. When finished, close the Command Prompt window and the Activate Windows window. If prompted, select the Yes button. The Mac should reboot to macOS.

8. After booting to macOS, eject and remove the flash drive. Proceed from where you left off in Julius Pargac's procedure.

I do not have a MacBook Pro with a T2 chip to test the above steps. If you have problems, then post a comment and/or update your question.