If they are the older IDE (ATA/PATA) you should be able to put one in. I would not recommend buying one from Apple - not sure if they even sell standalone drives. You can pick up DVD burners very cheaply these days and the 3rd party ones tend to have better media support.
They're not always supported in apps like imovie, itunes etc but you can usually work around that problem easily.
You can tell what connector the drives use by the width of the cable - IDE is the wide kind, SATA is the small one.
I don't think an SATA drive will work - people have said you can get it to work as the Mac Pro uses SATA for the hard drives but it's not easy to do. The manual suggests that it's still the older connection they use:http://manuals.info.apple.com/en/Mac...lDrive_DIY.pdf
So if you do buy another optical drive, don't buy an SATA one.