How many drives are you planning to connect? If you're trying to enable all 4 Mac Pro drive sleds, this card is probably not the way to go.
I'm guessing that you are looking for a PORT MULTIPLIER – not a SAS cable... Your RocketRAID 620 only has 2 SATA ports -- but it will support 5:1 port multipliers on each port (for a total of 10 supported drives). The Apple cable in your Mac Pro case is NOT a port multiplier cable... It is a SAS to SATA breakout cable -- there are actually 4 channels of full-speed SAS/SATA in that cable.
So, IF ** BIG IF ** you wanted to use this card with the internal Mac Pro drive sleds, you would need to first get a port multiplier (1 SATA-in to 4 or 5 SATA-PM-out). The port multiplier is never just a cable -- it will require a small circuit board. Then, you would use the same hydra cable that I used to connect the 4 SATA-PM-out ports to the SAS barrel. Then connect the Apple SAS/SATA breakout cable to the other end of the barrel. This would be an even bigger mess than what is inside my machine...
Alternatively, you could skip the port multiplier, but then you could only connect 2 drives to your card.
Let me know if that makes sense...
In my build, I used the SATA RAID ports on my motherboard (and turned off the RAID functionality in the BIOS). So each one of my drives mounts individually to the MacOS.
Hope that helps.
EDIT: I almost neglected the simple option -- of course, you could delete the Apple hot-swap cable all together... Simple SATA right-angle connectors on the back of the drives would work fine too. Still an issue if you want all 4 drives working...