Can i add a SATA Raid controller card to a server that already has a SCSI Raid controller card?

The 2800 only supports SCSI drives, so 500GB (SATA/ATA/SAS) drives will not work in this system.

I'd suggest using an imaging backup-tool (e.g. Acronis True Image Server), make a backup image onto a network share, then shut down the server, pull the drives out (mark where they came from). Put in new (larger) drives, create the new raid setup in the raid controller bios, then boot to the recovery media (Acronis home versions allow creating a bootable CD to use to restore an image, so I'd imaging that the server version offers the same feature), and restore the server from the image that you stored on the network share.

I put SATA cards (mostly Silicon Image-based PCI 3112 and PCI-Express 3132 cards) into every Dell server I work with, using them to control removable SATA drives for backups. Unless a compatibility issue exists between the 2800's BIOS and a particular RAID card BIOS, there's no reason you shouldn't be able to install a SATA RAID card.

Note, though, that there CAN be BIOS compatibility issues with particular SATA RAID controllers. For instance, I found that a Dell SC1600 wouldn't work with a high-end 3Ware PCI-X SATA RAID card, while it worked fine with a similarly-priced Areca PCI-X card. I've also used a $200 HighPoint SATA RAID card in a SC1600 server that needed more room for Exchange.

The 2800 has a drive backplane which accepts only SCA-80 drives.

If you can figure out a way to mount SATA drives and get power and SATA cables to them, you can try non-Dell raid controllers.