The conversion of a SCSI chassis DL380 G4 to the SAS chassis is possible, but not at all practical. It requires a bezel change, backplane modification, a new drive cage, a Smart Array P600 RAID controller (or SAS/SATA HBA) and will be severely crippled in performance. In addition, it can only accommodate 2.5" small-form-factor 1st generation SAS and SATA disks. You're also limited to 12GB of RAM.
The resulting face of the server would look like:
This model was sold between 2004 and 2006, and bridged the transition from legacy technologies like parallel SCSI and PCI-X to SAS/SATA and PCIe. This was also an early-generation 64-bit capable (EM64t) CPU.
I would not recommend this system for NexentaStor. NexentaStor will work in a limited capacity if you use this as a SCSI unit, though. It's really a bad idea to start any project on a 6 year-old server.
A better solution would be to find a G5 or greater ProLiant server that comes with native SAS/SATA functionality. If you must use the DL380 G4, you can find used 3.5" SCSI hard disks in capacities up to 300GB.
HP Proliant DL380 G4 - Can this server still perform in 2011?