Short answer is no, you can't.
The XServe RAID has IDE drives mounted in a carrier. The maximum size drive shipped by Apple, supported under 1.5.1 firmware on the controllers is 750GB, though finding these on the used market is farily difficult. Aside from that, any IDE drive can be put in the carriers. If your unit has 'blank' slots, these are indeed just blank slots - you will need to get carriers / drive modules if you want disks in the slot if that makes sense.
People on the Internet, myself included, have experimented down the route of using IDE to SATA bridges in the carriers with 2.5" SATA drives which are more freely available. This way you can utilise cheap drives in your drive modules, or even SSDs (which offer little benefit I have found). Some users have had success with 1TB 2.5" SATA drives in this method, but not with anything bigger it seems. Myself, I've had suceess with 500GB SATA and 256GB SSD with this hacky method.
Hope that helps.