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The Mac Pro only has PCI express slots, which is a modern serial based
standard. PCI is an old parallel based standard still found (for legacy
purposes) in many pcs but not in Macs.

There is no way to convert between the two (short of designing your
own hardware of course! ;))
 
There would be the issue with OS X drivers for the riser and the eSATA card. Questionable if that works out.

You also have to get Molex power from the optical drive bay via a cable through the bulk head hole. Not an easy retrofit but it can be done.
 
There's an inexpensive SIL3132 based PCIe SATA/eSATA card available on eBay UK (here, $2.80GPB). Certainly cheap enough. :D

It only operates as a 2 port card, and you can use the jumpers to configure internal or external ports. Drivers for it can be found on Silicon Image's site (work in SL too; OS X driver page). It can't boot OS X, but it does provide Hot Swap in the drivers. Hot Plug would be satisfied with an external drive enclosure, as it has it's own PSU. :)
 
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