SCO treats all host adapters that do not use the standard ide interface as SCSI.
When you run mkdev hd, choose SCSI, then you will be prompted for the name of the host adapter, with the default being the name of last device added (will be whatever the hd device is, if you haven't added a tape drive in the meantime.)
After that enter the scsi information.
You can see the current scsi information for the root drive in /etc/conf/cf.d/mscsi