Can an Amstrad XT be fitted with a CD drive?
Yes, but it isn't as easy as it looks. You will need:
An 8-bit SCSI controller.
The most common ones seem to be the Seagate ST-01/ST-02, and the Future Domain TMC-8xx series. In my experience, it's impossible to get the Seagate controllers working with anything other than very small Seagate hard drives, so that leaves you with the Future Domain controllers. These turn up on eBay now and again, sometimes in "SCSI CDROM kits".
Beware of QIC tape controllers, which have the same sort of connectors as SCSI controllers but are in fact completely different. Usually, a QIC controller has lots of little chips on it, while a SCSI controller has one or two big ones.
One other consideration: Some SCSI controllers have a BIOS ROM; some don't. If you want to have a SCSI hard drive as well as a CDROM, you need the BIOS ROM. [TMC-850 note]
A SCSI CDROM drive. This can either be internal (if you've got a spare 5.25" drive bay) or external. If the drive is external, you will need a cable of the correct type; there are at least three, and two look the same but are actually different: