I'm sure windows activation will complain, but you should be able to dd your windows partition (or disk) over to the external disk, set up a bootloader (windows can do this, but something like grub or syslinux I know would work to hand off to the windows bootloader)

I don't know anything about bitlocker stuff, probably needs to be decrypted before this can work.

That's what I would try, even though it's not wrapped up in a single tool.