How can I add more sata ports to the motherboard (SE7520AF2 in the SC5300BRP chassis? Or is theere a different motherboard for this chassis with more sata ports?
There's no way to physically add additional ports to the motherboard itself. You'll likely need to add a SATA RAID controller. Looking at the Spare Parts & Config Guide at:
the only SATA controller validated with the SE7520AF2 is the SRCS28X. It's an 8 port SATA controller. For more inforrmation see:
However, the SRCS28X has been discontinued (not manufactured any more) by Intel as of Feb 2009. You might have a tough time finding one of those.
Alternatively, you could get a current production SATA controller, but you'd have to validate it with the SE7520AF2 yourself.