I am running a full-disk encrypted Ubuntu 12.10.
Can I install Windows alongside, adding it at the boot menu on this machine? If so, how?
Yes, encryption (LUKS) does not affect the ability to boot Windows as :
Are you having difficulty or just asking ?
FWIW: if you are installing Windows second, you will need to restore grub after installing windows. See https://help.ubuntu.com/community/RecoveringUbuntuAfterInstallingWindows