While you cannot successfully install Windows directly on a USB external HDD, e.g., the docking station you mention, as Philip has indicated, you can - using a disk-cloning program - clone the contents of a bootable drive to a USBEHD and the latter device will happily boot.
I don't know if that's practical in your situation or precisely fits in with what you're looking for, but I thought I'd mention that possibility. I should just add that not every disk-cloning program possesses that capability - the one I work with does.
(As an aside, from time to time one comes across "instructions" (quite detailed & usually tortuous) on the net re directly installing a Windows OS onto a USBEHD. It's never worked for us, nor have I ever seen it successfully done.)