I suspect that you went through this when you created your current external SSD.
In reality, the BootCamp Assistant app is simply a script that manages Disk Utility to resize OS X and create a partition which it formats and then starts the Windows installation. In the process it creates a flash drive with the Windows installation and the Mac drivers and then boots to that. It does not allow for installing on an external drive to avoid some problems.
However, you can do the same thing using Disk Utility yourself (while running on your newly created OS X SSD) and resize your OS X while creating a new partition for Windows. Then format the new partition as FAT and use WinClone to restore the image you created earlier. WinClone can resize the Windows to utilize the additional disk space if you created a larger partition then you previously had. You probably don't need to reapply the Mac drivers since they were already a part of your old Windows installation.