At this time, the BeOS does not provide any support for RAID in software. If you have a RAID device set up by a software RAID toolkit, such as the FWB RAID Toolkit, it will not be accessible under the BeOS.
However, if you happen to have a hardware RAID device which pretends to be a single hard drive, that should work fine. These are rare, though.
We do intend to support RAID in a future BeOS release, but we do not have a schedule for that at the moment.