The optical drives don't run off a SATA connection.
There are usually companies out there that invent ways to cram more drives and connectors into the towers, but I suggest getting a SATA PCI card and an external SATA drive. Much easier and probably works better than puttnig even more heat-producing drives in your computer.
There are many options but G-Tech is one I know of:
I think the card with an internal connection is how they do it.
I've seen sites that sell mounts and braces for holding them into the case, but all I've seen is for the G5 tower. I haven't seen any for the MacPro yet, but I'm sure they're out there if you look long enough!