You can either remove the raid card, or create a collection of raid0 arrays, one for each drive. I had to do this with a perc because FreeNAS wouldn't see it as a JBOD/passthrough.
If its only 32bit, you're out of luck for ZFS. With the cost of a 2850 or 2950 being so cheap, I'd probably get something a little newer, although the 2950 will still only go up to 32G.