Well, the easiest way is to use an external casing for your cd-rom drive (I use a 4 bay SCSI-tower for my external equipment). These can be had with all types of internal and external SCSI-connectors. Simply take one which has a 50 pins cable on the inside and a Centronicstype SCSI-connector on the outside, complete with cable to hook it up to your A590.
Now about the cd-rom not working on your A590: I seem to remember that you can't use both the XT and SCSI-connector at the same time. If you want to use the SCSI-connector for your cd-romdrive, I expect you to have to replace your XT-drive with a SCSI one. Which is a very good idea anyway, as it will be much faster and have a much larger capacity...
BTW: you need to change dipswitchsettings for this.
