And if you are asking, if you can move your Windows 7 Virtual Machine (Windows is an Operating System and not a program) to an external HDD, then yes.
Just use Finder and go to your Home Directory (CMD+SHIFT+H) and find the Virtual Machine folder, it should reside in your Documents folder. From there copy the Windows 8 file(s) to the external HDD and after that is done, double click the copied file (when on your external HDD) to verify it works. After that, delete the VM in your Documents folder.
Do you employ any kind of backup strategy?