You can put a Powerbook SuperDrive in a Slot-loading iMac, if the SuperDrive is from a 400MHz-667MHz Titanium Powerbook G4.

If the Superdrive is from a later Powerbook, then it will be a standard laptop optical drive, and therefore will not fit in the iMac G3, which, as Silicon Valley Pirate says, uses the non-standard type of slot loading optical drive.

Best,

Matt