Optical drives (CD-ROM, DVD-ROM, DVD-RAM, DVD-R, CD-RW) are 5.25-inch devices and cannot physically fit in the 3.5-inch Zip drive bay and are also not supported.
Additional hard drives are intended to be mounted on the bottom of the case and require a SCSI card. Only Power Macintosh G3 (Blue and White) and Power Mac G4 computers with the U bracket in the hard drive expansion bay and a dual connector IDE cable can support two IDE drives in master and slave configuration. If the computer did not come with the dual connector IDE cable and U bracket, then additional hard drives are intended to be mounted on the bottom of the case and require a SCSI card.
For further information, refer to article 58193: "Power Macintosh G3 (Blue and White): Additional Hard Drive Installation Options".