This is absolutely do-able and not too hard, just a little patience. I've done it myself on a few 2850's that we have.
I modified slots 0-3 to accept SATA drives. I have 1TB drives in them with ZFS, raidz2.
Slots 4-5 were kept as is so I could RAID1 them with Nexenta.
The cost is under $40, not including stock trays or SATA card. You will have to do a little soldering to get the power to the drives, a little trimming on the alignment post of the SCSI connectors, and a little epoxy for the SATA+Power connector. Power is taken from each SCSI connector. With this mod, you can even hot swap the SATA drives without any problems whatsoever.
Note, you don't need the 3.3V feeds.
I'll see if I can put together a how-to. Meanwhile, here's the parts list.
Left-angle sata adapter, 1/2 meter, from estore.circuitassembly.com, P/N SKU:U709090. One each for each SATA drive. The depth of this connector is very important. Do not buy them from other vendors unless they look exactly like the one shown on the website. If you convert slots 4 and 5, then you will need a SATA extension cable for them.
Epoxy. Use any available that will harden and is resistance to some heat. Do not use conductive epoxy. I use a two part epoxy, but can't recall the exact type off-hand.
Drive trays. Use any available except P/N 0D988. This specific tray does not have the required alternate drive mounting holes. This is important to make the tray fit perfectly in the slot.
SATA interface card. I use AOC-SAT2-MV8. Works great.
When and if I have time, I'll figure out how to get the power and activity LEDs to light up too!